Integrated carbon capture and chemical production using fuel cells

ABSTRACT

In various aspects, systems and methods are provided for operating a molten carbonate fuel cell, such as a fuel cell assembly, with increased production of syngas or hydrogen while also reducing or minimizing the amount of CO 2  exiting the fuel cell in the cathode exhaust stream. This can allow for improved efficiency of syngas production while also generating electrical power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Ser. Nos. 61/787,587,61/787,697, 61/787,879, and 61/788,628, all filed on Mar. 15, 2013, eachof which is incorporated by reference herein in its entirety. Thisapplication also claims the benefit of U.S. Ser. Nos. 61/884,376,61/884,545, 61/884,565, 61/884,586, 61/884,605, and 61/884,635, allfiled on Sep. 30, 2013, each of which is incorporated by referenceherein in its entirety. This application further claims the benefit ofU.S. Ser. No. 61/889,757, filed on Oct. 11, 2013, which is incorporatedby reference herein in its entirety.

This application is related to 25 other co-pending U.S. applications,filed on even date herewith, and identified by the following AttorneyDocket numbers and titles: 2013EM104-US2 entitled “Integrated PowerGeneration and Carbon Capture using Fuel Cells”; 2013EM104-US3 entitled“Integrated Power Generation and Carbon Capture using Fuel Cells”;2013EM107-US2 entitled “Integrated Power Generation and Carbon Captureusing Fuel Cells”; 2013EM107-US3 entitled “Integrated Power Generationand Carbon Capture using Fuel Cells”; 2013EM108-US2 entitled “IntegratedPower Generation and Carbon Capture using Fuel Cells”; 2013EM108-US3entitled “Integrated Power Generation and Carbon Capture using FuelCells”; 2013EM109-US2 entitled “Integrated Power Generation and CarbonCapture using Fuel Cells”; 2013EM109-US3 entitled “Integrated PowerGeneration and Carbon Capture using Fuel Cells”; 2013EM272-US2 entitled“Integrated Power Generation and Chemical Production using Fuel Cells”;2013EM273-US2 entitled “Integrated Power Generation and ChemicalProduction using Fuel Cells at a Reduced Electrical Efficiency”;2013EM274-US2 entitled “Integrated Power Generation and ChemicalProduction using Fuel Cells”; 2013EM277-US2 entitled “Integrated PowerGeneration and Chemical Production using Fuel Cells”; 2013EM279-US2entitled “Integrated Power Generation and Chemical Production using FuelCells”; 2013EM285-US2 entitled “Integrated Operation of Molten CarbonateFuel Cells”; 2014EM047-US entitled “Mitigation of NOx in IntegratedPower Production”; 2014EM048-US entitled “Integrated Power Generationusing Molten Carbonate Fuel Cells”; 2014EM049-US entitled “Integrated ofMolten Carbonate Fuel Cells in Fischer-Tropsch Synthesis”; 2014EM050-USentitled “Integrated of Molten Carbonate Fuel Cells in Fischer-TropschSynthesis”; 2014EM051-US entitled “Integrated of Molten Carbonate FuelCells in Fischer-Tropsch Synthesis”; 2014EM052-US entitled “Integratedof Molten Carbonate Fuel Cells in Methanol Synthesis”; 2014EM053-USentitled “Integrated of Molten Carbonate Fuel Cells in a RefinerySetting”; 2014EM054-US entitled “Integrated of Molten Carbonate FuelCells for Synthesis of Nitrogen Compounds”; 2014EM055-US entitled“Integrated of Molten Carbonate Fuel Cells with Fermentation Processes”;2014EM056-US entitled “Integrated of Molten Carbonate Fuel Cells in Ironand Steel Processing”; and 2014EM057-US entitled “Integrated of MoltenCarbonate Fuel Cells in Cement Processing”. Each of these co-pendingU.S. applications is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

In various aspects, the invention is related to chemical productionand/or power generation processes integrated with electrical powerproduction using molten carbonate fuel cells.

BACKGROUND OF THE INVENTION

Molten carbonate fuel cells utilize hydrogen and/or other fuels togenerate electricity. The hydrogen may be provided by reforming methaneor other reformable fuels in a steam reformer that is upstream of thefuel cell or within the fuel cell. Reformable fuels can encompasshydrocarbonaceous materials that can be reacted with steam and/or oxygenat elevated temperature and/or pressure to produce a gaseous productthat comprises hydrogen. Alternatively or additionally, fuel can bereformed in the anode cell in a molten carbonate fuel cell, which can beoperated to create conditions that are suitable for reforming fuels inthe anode. Alternately or additionally, the reforming can occur bothexternally and internally to the fuel cell.

Traditionally, molten carbonate fuel cells are operated to maximizeelectricity production per unit of fuel input, which may be referred toas the fuel cell's electrical efficiency. This maximization can be basedon the fuel cell alone or in conjunction with another power generationsystem. In order to achieve increased electrical production and tomanage the heat generation, fuel utilization within a fuel cell istypically maintained at 70% to 75%.

U.S. Published Patent Application 2011/0111315 describes a system andprocess for operating fuel cell systems with substantial hydrogencontent in the anode inlet stream. The technology in the '315publication is concerned with providing enough fuel in the anode inletso that sufficient fuel remains for the oxidation reaction as the fuelapproaches the anode exit. To ensure adequate fuel, the '315 publicationprovides fuel with a high concentration of H₂. The H₂ not utilized inthe oxidation reaction is recycled to the anode for use in the nextpass. On a single pass basis, the H₂ utilization may range from 10% to30%. The '315 reference does not describe significant reforming withinthe anode, instead relying primarily on external reforming.

U.S. Published Patent Application 2005/0123810 describes a system andmethod for co-production of hydrogen and electrical energy. Theco-production system comprises a fuel cell and a separation unit, whichis configured to receive the anode exhaust stream and separate hydrogen.A portion of the anode exhaust is also recycled to the anode inlet. Theoperating ranges given in the '810 publication appear to be based on asolid oxide fuel cell. Molten carbonate fuel cells are described as analternative.

U.S. Published Patent Application 2003/0008183 describes a system andmethod for co-production of hydrogen and electrical power. A fuel cellis mentioned as a general type of chemical converter for converting ahydrocarbon-type fuel to hydrogen. The fuel cell system also includes anexternal reformer and a high temperature fuel cell. An embodiment of thefuel cell system is described that has an electrical efficiency of about45% and a chemical production rate of about 25% resulting in a systemcoproduction efficiency of about 70%. The '183 publication does notappear to describe the electrical efficiency of the fuel cell inisolation from the system.

U.S. Pat. No. 5,084,362 describes a system for integrating a fuel cellwith a gasification system so that coal gas can be used as a fuel sourcefor the anode of the fuel cell. Hydrogen generated by the fuel cell isused as an input for a gasifier that is used to generate methane from acoal gas (or other coal) input. The methane from the gasifier is thenused as at least part of the input fuel to the fuel cell. Thus, at leasta portion of the hydrogen generated by the fuel cell is indirectlyrecycled to the fuel cell anode inlet in the form of the methanegenerated by the gasifier.

An article in the Journal of Fuel Cell Science and Technology (G.Manzolini et. al., J. Fuel Cell Sci. and Tech., Vol. 9, February 2012)describes a power generation system that combines a combustion powergenerator with molten carbonate fuel cells. Various arrangements of fuelcells and operating parameters are described. The combustion output fromthe combustion generator is used in part as the input for the cathode ofthe fuel cell. One goal of the simulations in the Manzolini article isto use the MCFC to separate CO₂ from the power generator's exhaust. Thesimulation described in the Manzolini article establishes a maximumoutlet temperature of 660° C. and notes that the inlet temperature mustbe sufficiently cooler to account for the temperature increase acrossthe fuel cell. The electrical efficiency (i.e. electricitygenerated/fuel input) for the MCFC fuel cell in a base model case is50%. The electrical efficiency in a test model case, which is optimizedfor CO₂ sequestration, is also 50%.

An article by Desideri et al. (Intl. J. of Hydrogen Energy, Vol. 37,2012) describes a method for modeling the performance of a powergeneration system using a fuel cell for CO₂ separation. Recirculation ofanode exhaust to the anode inlet and the cathode exhaust to the cathodeinlet are used to improve the performance of the fuel cell. The modelparameters describe an MCFC electrical efficiency of 50.3%.

SUMMARY OF THE INVENTION

In an aspect, a method for producing, with low CO₂ emissions,electricity and hydrogen or syngas using a molten carbonate fuel cellhaving an anode and cathode is provided. The method includes introducinga fuel stream comprising a reformable fuel into an anode of a moltencarbonate fuel cell, a reforming stage associated with the anode, or acombination thereof; introducing a cathode inlet stream comprising CO₂and O₂ into a cathode of the fuel cell; generating electricity withinthe molten carbonate fuel cell; and withdrawing, from an anode exhaust,a gas stream comprising H₂, a gas stream comprising H₂ and CO, or acombination thereof, wherein a fuel utilization in the anode is about50% or less and a CO₂ utilization in the cathode is at least about 60%.Optionally, a ratio of the reformable hydrogen content of the reformablefuel in the fuel stream relative to an amount of hydrogen reacted in theanode can be at least about 1.0, such as at least about 2.0. Optionally,the gas stream comprising CO₂ and O₂ comprises about 20 vol % CO₂ orless, such as about 15 vol % or less, or about 12 vol % or less, orabout 10 vol % or less.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows an example of a configuration for moltencarbonate fuel cells and associated reforming and separation stages.

FIG. 2 schematically shows another example of a configuration for moltencarbonate fuel cells and associated reforming and separation stages.

FIG. 3 schematically shows an example of the operation of a moltencarbonate fuel cell.

FIG. 4 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIG. 5 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIG. 6 schematically shows an example of a balanced configuration foroperating a molten carbonate fuel cell.

FIGS. 7-10 show simulation data from various configurations foroperating a molten carbonate fuel cell.

FIG. 11 shows a graph of a simulated ratio of net moles of syngas in theanode exhaust to moles of CO₂ in a cathode exhaust at different fuelutilizations for benzene and CH₄.

FIGS. 12 and 13 show examples of CH₄ conversion at different fuel celloperating voltages V_(A).

DETAILED DESCRIPTION OF THE EMBODIMENTS Overview

Prior art systems can typically seek to maximize various parameters,most prominently electrical energy efficiency, in the operation ofmolten carbonate fuel cell systems. In such fuel cells, CO₂ can beintroduced as a reactant into the cathode, and some of the CO₂ can bereacted with oxygen in the cathode. This reaction is explained below inmore detail. Because of the CO₂ reaction, less CO₂ may be present in thecathode effluent than in the cathode influent. In typical operations,the amount of CO₂ remaining in the cathode effluent can be kept as highas possible to maximize the electrical efficiency and electrical poweroutput of the MCFC. A high CO₂ content in the effluent is consistentwith typical MCFC operation, which seeks to produce maximum electricityand only enough combustible anode effluent gas to provide heat forheating the reactants to the operational temperature of the fuel cell.

In various aspects, systems and methods are provided for operating amolten carbonate fuel cell (such as a fuel cell assembly) with low fuelutilization but high CO₂ utilization. CO₂ exiting the cathode may beexhausted to the atmosphere and can be undesirable where CO₂ emissionsare considered environmentally detrimental, such that, in someembodiments, the cathode exhaust is further treated before certain ofits components are emitted to the atmosphere. Thus, controlling CO₂emissions through the use of high CO₂ utilization can be desirable insome operations. Carbon, in the form of CO₂ can be transported acrossthe electrolyte membrane in the fuel cell and then reacted with hydrogento become part of the syngas mixture (H₂, CO, CO₂ and H₂O) in the anode.This can allow for improved carbon capture, but can result in low orreduced electrical efficiency. However, this type of operation can stilllead to an improved total fuel cell efficiency based on the productionof excess syngas and/or hydrogen generated by the fuel cell (optionallyincluding an associated reforming stage).

In a system for using MCFCs for carbon capture (where capture may refer,in general, to the removal of CO₂ from the cathode inlet stream for usein other processes), the operating conditions for the MCFC can beselected based on a balance between improved transfer of CO₂ fromcathode to anode and improved electrical efficiency. In such a system,the fuel utilization can be between about 70% and about 75%, or even ashigh as about 80% if achievable, in order to maximize the amount ofelectrical energy generated while maintaining a reasonable thermalgradient across the fuel cell. In this type of configuration, theremaining fuel in the anode exhaust can be combusted to provide part ofthe heat for operation of the fuel cell. A fuel utilization of about 75%corresponds to having total reformable hydrogen content of all fueldelivered to the anode (and/or to a reforming stage associated with theanode) of about 33% greater than the amount of fuel oxidized in theanode.

By contrast, in various aspects a molten carbonate fuel cell can beoperated at a reduced fuel utilization value, such as a fuel utilizationof about 50% or less, while also having a high CO₂ utilization value,such as at least about 60%. In this type of configuration, the moltencarbonate fuel cell can be effective for carbon capture, as the CO₂utilization can advantageously be sufficiently high. Rather thanattempting to maximize electrical efficiency, in this type ofconfiguration the total efficiency of the fuel cell can be improved orincreased based on the combined electrical and chemical efficiency. Thechemical efficiency can be based on withdrawal of a hydrogen and/orsyngas stream from the anode exhaust as an output for use in otherprocesses. Even though the electrical efficiency may be reduced relativeto some conventional configurations by forming additionalhydrogen/syngas, making use of the chemical energy output in the anodeexhaust can allow for a desirable total efficiency for the fuel cell.

In various aspects, the fuel utilization in the fuel cell anode can beabout 50% or less, such as about 40% or less, or about 30% or less, orabout 25% or less, or about 20% or less. Additionally or alternately, inorder to generate at least some electric power, the fuel utilization inthe fuel cell can be at least about 5%, such as at least about 10%, orat least about 15%, or at least about 20%, or at least about 25%, or atleast about 30%. Further additionally or alternatively, the CO₂utilization can be at least about 60%, such as at least about 65%, or atleast about 70%, or at least about 75%. Still further additionally oralternately, the CO₂ utilization can be about 98% or less, oralternatively can be low enough so that sufficient CO₂ can remain in thecathode exhaust to allow operation of the fuel cell. As used herein, CO₂utilization may be the difference between the moles of CO₂ in thecathode outlet stream and the moles of CO₂ in the cathode inlet streamdivided by the moles of CO₂ in the cathode inlet. Expressedmathematically, CO₂utilization=(CO_(2(cathode in))−CO_(2(cathode out)))/CO_(2(cathode in)).

One option for operating a molten carbonate fuel cell with a relativelylow fuel utilization can be to have a large excess of reformable fuelthat can be reformed relative to the amount of reaction of hydrogen inthe anode. Instead of operating the fuel cell with a large amount ofrecycle of anode exhaust to the anode inlet, an excess of reformablefuel can be used, and the reformable fuel can be substantially reformedin the fuel cell or an associated reforming stage. This allows forproduction of excess hydrogen and/or synthesis gas that can correspondto a chemical energy output from the fuel cell. In various aspects, aratio of the reformable hydrogen content of the reformable fuel in thefuel stream relative to an amount of hydrogen reacted in the anode canbe at least about 1.5:1, or at least about 2.0:1, or at least about2.5:1, or at least about 3.0:1. Additionally or alternately, the ratioof reformable hydrogen content of the reformable fuel in the fuel streamrelative to the amount of hydrogen reacted in the anode can be about20:1 or less, such as about 15:1 or less or about 10:1 or less. In oneaspect, it is contemplated that less than 100% of the reformablehydrogen content in the anode inlet stream can be converted to hydrogen.For example, at least about 80% of the reformable hydrogen content in ananode inlet stream can be converted to hydrogen in the anode and/or inan associated reforming stage(s), such as at least about 85%, or atleast about 90%.

Hydrogen and/or syngas can be withdrawn from the anode exhaust as achemical energy output. Hydrogen can be used as a clean fuel withoutgenerating greenhouse gases when it is burned or combusted. Instead, forhydrogen generated by reforming of hydrocarbons (or hydrocarbonaceouscompounds), the CO₂ can be considered to have already been “captured” inthe anode loop. Additionally or alternately, hydrogen can be a valuableinput for a variety of refinery processes and/or other synthesisprocesses. Syngas can also be a valuable input for a variety ofprocesses. In addition to having fuel value, syngas can be used as afeedstock for producing other higher value products, such as by usingsyngas as an input for Fischer-Tropsch synthesis and/or methanolsynthesis processes.

In various aspects, the anode exhaust can have a ratio of H₂ to CO ofabout 1.5:1 to about 10:1, such as at least about 3.0:1, or at leastabout 4.0:1, or at least about 5.0:1, and/or about 8.0:1 or less, orabout 6.0:1 or less. A syngas stream can be withdrawn from the anodeexhaust. In various aspects, a syngas stream withdrawn from an anodeexhaust can have a ratio of moles of H₂ to moles of CO of at least about0.9:1, such as at least about 1.0:1, or at least about 1.2:1, or atleast about 1.5:1, or at least about 1.7:1, or at least about 1.8:1, orat least about 1.9:1. Additionally or alternately, the molar ratio of H₂to CO in a syngas withdrawn from an anode exhaust can be about 3.0:1 orless, such as about 2.7:1 or less, or about 2.5:1 or less, or about2.3:1 or less, or about 2.2:1 or less, or about 2.1:1 or less. It isnoted that higher ratios of H₂ to CO in a withdrawn syngas stream cantend to reduce the amount of CO relative to the amount of CO₂ in acathode exhaust. However, many types of syngas applications benefit fromsyngas with a molar ratio of H₂ to CO of at least about 1.5:1 to about2.5:1 or less, so forming a syngas stream with a molar ratio of H₂ to COcontent of, for example, about 1.7:1 to about 2.3:1 may be desirable forsome applications.

Syngas can be withdrawn from an anode exhaust by any convenient method.In some aspects, syngas can be withdrawn from the anode exhaust byperforming separations on the anode exhaust to remove at least a portionof the components in the anode exhaust different from H₂ and CO. Forexample, an anode exhaust can first be passed through an optionalwater-gas shift stage to adjust the relative amounts of H₂ and CO. Oneor more separation stages can then be used to remove H₂O and/or CO₂ fromthe anode exhaust. The remaining portion of the anode exhaust can thencorrespond to the syngas stream, which can then be withdrawn for use inany convenient manner. Additionally or alternately, the withdrawn syngasstream can be passed through one or more water-gas shift stages and/orpassed through one or more separation stages.

It is noted that an additional or alternative way of modifying the molarratio of H₂ to CO in the withdrawn syngas can be to separate an H₂stream from the anode exhaust and/or the syngas, such as by performing amembrane separation. Such a separation to form a separate H₂ outputstream can be performed at any convenient location, such as prior toand/or after passing the anode exhaust through a water-gas shiftreaction stage, and/or prior to and/or after passing the anode exhaustthrough one or more separation stages for removing components in theanode exhaust different from H₂ and CO. Optionally, a water-gas shiftstage can be used before, after, or both before and after separation ofan H₂ stream from the anode exhaust. In an additional or alternativeembodiment, H₂ can optionally be separated from the withdrawn syngasstream. In some aspects, a separated H₂ stream can correspond to a highpurity H₂ stream, such as an H₂ stream containing at least about 90 vol% of H₂, such as at least about 95 vol % of H₂ or at least about 99 vol% of H₂.

A potential advantage of a molten carbonate fuel cell can be the abilityto receive CO₂ from a high flow volume stream (such as an exhaust flowfrom a combustion generator) on the cathode side of the fuel cell andtransfer the CO₂ to a lower volume flow passing through the anodeportion of the fuel cell. This can facilitate capture of the CO₂ forsubsequent sequestration and/or use in another process, such as enhancedoil recovery. By using molten carbonate fuel cells, a process forgeneration of syngas can be integrated with a process for capturing CO₂.This can allow for improved total efficiency (electrical plus chemical)for the fuel cell while also reducing or minimizing the amount of CO₂emitted as part of a CO₂-containing stream, such as a combustion exhauststream. This can advantageously be in contrast to conventional schemesfor using a molten carbonate fuel cell for carbon capture. Suchconventional schemes typically attempt to maximize electric efficiency,and therefore often explicitly avoid generating a chemical energy output(such as syngas) for use in an external process.

In some aspects, a molten carbonate fuel cell can be operated with ahigh CO₂ utilization using a cathode input feed with a moderate or lowCO₂ content. A variety of streams desirable for carbon separation andcapture can include streams with moderate to low CO₂ content. Forexample, a potential input stream for a cathode inlet can have a CO₂content of about 20 vol % or less, such as about 15 vol % or less, orabout 12 vol % or less, or about 10 vol % or less. Such a CO₂-containingstream can be generated by a combustion generator, such as a coal-firedor natural gas-fired turbine. Achieving a desired level of CO₂utilization on a cathode input stream with a moderate or low CO₂ contentcan allow for use of a lower content CO₂ stream, as opposed to needingto enrich a stream with CO₂ prior to using the stream as a cathode inputstream.

Additionally or alternately, operating a molten carbonate fuel cell athigh CO₂ utilization values and/or with a cathode inlet stream having alow CO₂ content can assist with achieving a desired ratio of net syngasin the anode exhaust relative to carbon in the cathode exhaust. As apractical matter, there can be limitations on the amount of CO₂ that canbe removed in a cathode of a commercial scale molten carbonate fuel cellwhile still operating the fuel cell in a desirable range. In variousaspects, the cathode exhaust from a molten carbonate fuel cell can havea CO₂ content of at least about 0.5%, such as at least about 1.0 vol %,or at least about 1.5 vol %, or at least about 2.0 vol %. Due to thislimitation, the amount of CO₂ in the cathode exhaust cannot be reducedto a value close to zero. This means that a high ratio of carbonwithdrawn as syngas versus carbon in the cathode exhaust cannot beachieved simply by starting with a low carbon-content cathode inputstream and then reducing the carbon content close to zero in thecathode. Instead, a desired ratio can be achieved by increasing the CO₂utilization in the cathode, so that a substantial portion of the CO₂passed into the cathode can be transferred from the cathode to the anodein the fuel cell. Depending on the aspect, the CO₂ utilization for thefuel cell can be at least about 33%, such as at least about 40%, or atleast about 50%, or at least about 60%, or at least about 70%.

In some aspects, the CO₂ content of the input stream for the cathode canbe a low CO₂ content stream, such as a combustion exhaust stream from anatural gas turbine. Such low CO₂ content streams can have a CO₂concentration of about 10 vol % or less, or about 9 vol % or less, orabout 8 vol % or less, or about 7 vol % or less, or about 6.5 vol % orless, or about 6 vol % or less, or about 5 vol % or less, or about 4.5vol % or less. Alternatively, the CO₂ content of the input stream forthe cathode can be about 1.5 vol % or greater, or about 1.6 vol % orgreater, or about 1.7 vol % or greater, or about 1.8 vol % or greater,or about 1.9 vol % or greater, or about 2 vol % or greater, or about 3vol % or greater. For this type of low CO₂ content stream as a cathodeinput, the minimum CO₂ content value in the cathode exhaust can place apractical limit on the CO₂ utilization. In such aspects, a lower ratioof CO in the withdrawn syngas stream to CO₂ in the cathode exhaust canbe acceptable, such as a ratio of at least about 0.5, or at least about0.75. Alternatively, for higher CO₂ content input streams to the cathodeinlet, higher ratios of CO in the withdrawn syngas stream to CO₂ in thecathode exhaust can be desirable, such as a ratio of at least about 1.0.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell canbe operated so that the amount of reforming can be selected relative tothe amount of oxidation in order to achieve a desired thermal ratio forthe fuel cell. As used herein, the “thermal ratio” is defined as theheat produced by exothermic reactions in a fuel cell assembly divided bythe endothermic heat demand of reforming reactions occurring within thefuel cell assembly. Expressed mathematically, the thermal ratio(TH)=Q_(Ex)/Q_(EN), where Q_(EX) is the sum of heat produced byexothermic reactions and Q_(EN) is the sum of heat consumed by theendothermic reactions occurring within the fuel cell. Note that the heatproduced by the exothermic reactions corresponds to any heat due toreforming reactions, water gas shift reactions, and the electrochemicalreactions in the cell. The heat generated by the electrochemicalreactions can be calculated based on the ideal electrochemical potentialof the fuel cell reaction across the electrolyte minus the actual outputvoltage of the fuel cell. For example, the ideal electrochemicalpotential of the reaction in a MCFC is believed to be about 1.04V basedon the net reaction that occurs in the cell. During operation of theMCFC, the cell will typically have an output voltage less than 1.04 Vdue to various losses. For example, a common output/operating voltagecan be about 0.7 V. The heat generated is equal to the electrochemicalpotential of the cell (i.e. ˜1.04V) minus the operating voltage. Forexample, the heat produced by the electrochemical reactions in the cellis ˜0.34 V when the output voltage of ˜0.7V. Thus, in this scenario, theelectrochemical reactions would produce ˜0.7 V of electricity and ˜0.34V of heat energy. In such an example, the ˜0.7 V of electrical energy isnot included as part of Q_(EX). In other words, heat energy is notelectrical energy.

In various aspects, a thermal ratio can be determined for any convenientfuel cell structure, such as a fuel cell stack, an individual fuel cellwithin a fuel cell stack, a fuel cell stack with an integrated reformingstage, a fuel cell stack with an integrated endothermic reaction stage,or a combination thereof. The thermal ratio may also be calculated fordifferent units within a fuel cell stack, such as an assembly of fuelcells or fuel cell stacks. For example, the thermal ratio may becalculated for a single anode within a single fuel cell, an anodesection within a fuel cell stack, or an anode section within a fuel cellstack along with integrated reforming stages and/or integratedendothermic reaction stage elements in sufficiently close proximity tothe anode section to be integrated from a heat integration standpoint.As used herein, “an anode section” comprises anodes within a fuel cellstack that share a common inlet or outlet manifold.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a thermal ratio. Where fuel cells are operatedto have a desired thermal ratio, a molten carbonate fuel cell can beoperated to have a thermal ratio of about 1.5 or less, for example about1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 orless, or about 0.75 or less. Additionally or alternately, the thermalratio can be at least about 0.25, or at least about 0.35, or at leastabout 0.45, or at least about 0.50. Additionally or alternately, in someaspects the fuel cell can be operated to have a temperature rise betweenanode input and anode output of about 40° C. or less, such as about 20°C. or less, or about 10° C. or less. Further additionally oralternately, the fuel cell can be operated to have an anode outlettemperature that is from about 10° C. lower to about 10° C. higher thanthe temperature of the anode inlet. Still further additionally oralternately, the fuel cell can be operated to have an anode inlettemperature that is greater than the anode outlet temperature, such asat least about 5° C. greater, or at least about 10° C. greater, or atleast about 20° C. greater, or at least about 25° C. greater. Yet stillfurther additionally or alternately, the fuel cell can be operated tohave an anode inlet temperature that is greater than the anode outlettemperature by about 100° C. or less, such as by about 80° C. or less,or about 60° C. or less, or about 50° C. or less, or about 40° C. orless, or about 30° C. or less, or about 20° C. or less.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated with an excess ofreformable fuel relative to the amount of hydrogen reacted in the anodeof the fuel cell. Instead of selecting the operating conditions of afuel cell to improve or maximize the electrical efficiency of the fuelcell, an excess of reformable fuel can be passed into the anode of thefuel cell to increase the chemical energy output of the fuel cell.Optionally but preferably, this can lead to an increase in the totalefficiency of the fuel cell based on the combined electrical efficiencyand chemical efficiency of the fuel cell.

In some aspects, the reformable hydrogen content of reformable fuel inthe input stream delivered to the anode and/or to a reforming stageassociated with the anode can be at least about 50% greater than theamount of hydrogen oxidized in the anode, such as at least about 75%greater or at least about 100% greater. In various aspects, a ratio ofthe reformable hydrogen content of the reformable fuel in the fuelstream relative to an amount of hydrogen reacted in the anode can be atleast about 1.5:1, or at least about 2.0:1, or at least about 2.5:1, orat least about 3.0:1. Additionally or alternately, the ratio ofreformable hydrogen content of the reformable fuel in the fuel streamrelative to the amount of hydrogen reacted in the anode can be about20:1 or less, such as about 15:1 or less or about 10:1 or less. In oneaspect, it is contemplated that less than 100% of the reformablehydrogen content in the anode inlet stream can be converted to hydrogen.For example, at least about 80% of the reformable hydrogen content in ananode inlet stream can be converted to hydrogen in the anode and/or inan associated reforming stage, such as at least about 85%, or at leastabout 90%.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated with increased productionof syngas (or hydrogen) while also reducing or minimizing the amount ofCO₂ exiting the fuel cell in the cathode exhaust stream. Syngas can be avaluable input for a variety of processes. In addition to having fuelvalue, syngas can be used as a raw material for forming other highervalue products, such as by using syngas as an input for Fischer-Tropschsynthesis and/or methanol synthesis processes. One option for makingsyngas can be to reform a hydrocarbon or hydrocarbon-like fuel, such asmethane or natural gas. For many types of industrial processes, a syngashaving a ratio of H₂ to CO of close to 2:1 (or even lower) can often bedesirable. A water gas shift reaction can be used to reduce the H₂ to COratio in a syngas if additional CO₂ is available, such as is produced inthe anodes.

One way of characterizing the overall benefit provided by integratingsyngas generation with use of molten carbonate fuel cells can be basedon a ratio of the net amount of syngas that exits the fuel cells in theanode exhaust relative to the amount of CO₂ that exits the fuel cells inthe cathode exhaust. This characterization measures the effectiveness ofproducing power with low emissions and high efficiency (both electricaland chemical). In this description, the net amount of syngas in an anodeexhaust is defined as the combined number of moles of H₂ and number ofmoles of CO present in the anode exhaust, offset by the amount of H₂ andCO present in the anode inlet. Because the ratio is based on the netamount of syngas in the anode exhaust, simply passing excess H₂ into theanode does not change the value of the ratio. However, H₂ and/or COgenerated due to reforming in the anode and/or in an internal reformingstage associated with the anode can lead to higher values of the ratio.Hydrogen oxidized in the anode can lower the ratio. It is noted that thewater gas shift reaction can exchange H₂ for CO, so the combined molesof H₂ and CO represents the total potential syngas in the anode exhaust,regardless of the eventual desired ratio of H₂ to CO in a syngas. Thesyngas content of the anode exhaust (H₂+CO) can then be compared withthe CO₂ content of the cathode exhaust. This can provide a type ofefficiency value that can also account for the amount of carbon capture.This can equivalently be expressed as an equation as

Ratio of net syngas in anode exhaust to cathode CO₂=net moles of(H₂+CO)_(ANODE)/moles of (CO₂)_(CATHODE)

In various aspects, the ratio of net moles of syngas in the anodeexhaust to the moles of CO₂ in the cathode exhaust can be at least about2.0, such as at least about 3.0, or at least about 4.0, or at leastabout 5.0. In some aspects, the ratio of net syngas in the anode exhaustto the amount of CO₂ in the cathode exhaust can be still higher, such asat least about 10.0, or at least about 15.0, or at least about 20.0.Ratio values of about 40.0 or less, such as about 30.0 or less, or about20.0 or less, can additionally or alternately be achieved. In aspectswhere the amount of CO₂ at the cathode inlet is about 6.0 volume % orless, such as about 5.0 volume % or less, ratio values of at least about1.5 may be sufficient/realistic. Such molar ratio values of net syngasin the anode exhaust to the amount of CO₂ in the cathode exhaust can begreater than the values for conventionally operated fuel cells.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can also be operated at conditions thatcan improve or optimize the combined electrical efficiency and chemicalefficiency of the fuel cell. Instead of selecting conventionalconditions for maximizing the electrical efficiency of a fuel cell, theoperating conditions can allow for output of excess synthesis gas and/orhydrogen in the anode exhaust of the fuel cell. The synthesis gas and/orhydrogen can then be used in a variety of applications, includingchemical synthesis processes and collection of hydrogen for use as a“clean” fuel. In aspects of the invention, electrical efficiency can bereduced to achieve a high overall efficiency, which includes a chemicalefficiency based on the chemical energy value of syngas and/or hydrogenproduced relative to the energy value of the fuel input for the fuelcell.

In some aspects, the operation of the fuel cells can be characterizedbased on electrical efficiency. Where fuel cells are operated to have alow electrical efficiency (EE), a molten carbonate fuel cell can beoperated to have an electrical efficiency of about 40% or less, forexample, about 35% EE or less, about 30% EE or less, about 25% EE orless, or about 20% EE or less, about 15% EE or less, or about 10% EE orless. Additionally or alternately, the EE can be at least about 5%, orat least about 10%, or at least about 15%, or at least about 20%.Further additionally or alternately, the operation of the fuel cells canbe characterized based on total fuel cell efficiency (TFCE), such as acombined electrical efficiency and chemical efficiency of the fuelcell(s). Where fuel cells are operated to have a high total fuel cellefficiency, a molten carbonate fuel cell can be operated to have a TFCE(and/or combined electrical efficiency and chemical efficiency) of about55% or more, for example, about 60% or more, or about 65% or more, orabout 70% or more, or about 75% or more, or about 80% or more, or about85% or more. It is noted that for a total fuel cell efficiency and/orcombined electrical efficiency and chemical efficiency, any additionalelectricity generated from use of excess heat generated by the fuel cellcan be excluded from the efficiency calculation.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired electrical efficiency of about 40%or less and a desired total fuel cell efficiency of about 55% or more.Where fuel cells are operated to have a desired electrical efficiencyand a desired total fuel cell efficiency, a molten carbonate fuel cellcan be operated to have an electrical efficiency of about 40% or lesswith a TFCE of about 55% or more, for example, about 35% EE or less withabout a TFCE of 60% or more, about 30% EE or less with about a TFCE ofabout 65% or more, about 25% EE or less with about a 70% TFCE or more,or about 20% EE or less with about a TFCE of 75% or more, about 15% EEor less with about a TFCE of 80% or more, or about 10% EE or less withabout a TFCE of about 85% or more.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated at conditions that canprovide increased power density. The power density of a fuel cellcorresponds to the actual operating voltage V_(A) multiplied by thecurrent density I. For a molten carbonate fuel cell operating at avoltage V_(A), the fuel cell also can tend to generate waste heat, thewaste heat defined as (V₀−V_(A))*I based on the differential betweenV_(A) and the ideal voltage V_(o) for a fuel cell providing currentdensity I. A portion of this waste heat can be consumed by reforming ofa reformable fuel within the anode of the fuel cell. The remainingportion of this waste heat can be absorbed by the surrounding fuel cellstructures and gas flows, resulting in a temperature differential acrossthe fuel cell. Under conventional operating conditions, the powerdensity of a fuel cell can be limited based on the amount of waste heatthat the fuel cell can tolerate without compromising the integrity ofthe fuel cell.

In various aspects, the amount of waste heat that a fuel cell cantolerate can be increased by performing an effective amount of anendothermic reaction within the fuel cell. One example of an endothermicreaction includes steam reforming of a reformable fuel within a fuelcell anode and/or in an associated reforming stage, such as anintegrated reforming stage in a fuel cell stack. By providing additionalreformable fuel to the anode of the fuel cell (or to anintegrated/associated reforming stage), additional reforming can beperformed so that additional waste heat can be consumed. This can reducethe amount of temperature differential across the fuel cell, thusallowing the fuel cell to operate under an operating condition with anincreased amount of waste heat. The loss of electrical efficiency can beoffset by the creation of an additional product stream, such as syngasand/or H₂, that can be used for various purposes including additionalelectricity generation further expanding the power range of the system.

In various aspects, the amount of waste heat generated by a fuel cell,(V₀−V_(A))*I as defined above, can be at least about 30 mW/cm², such asat least about 40 mW/cm², or at least about 50 mW/cm², or at least about60 mW/cm², or at least about 70 mW/cm², or at least about 80 mW/cm², orat least about 100 mW/cm², or at least about 120 mW/cm², or at leastabout 140 mW/cm², or at least about 160 mW/cm², or at least about 180mW/cm². Additionally or alternately, the amount of waste heat generatedby a fuel cell can be less than about 250 mW/cm², such as less thanabout 200 mW/cm², or less than about 180 mW/cm², or less than about 165mW/cm², or less than about 150 mW/cm².

Although the amount of waste heat being generated can be relativelyhigh, such waste heat may not necessarily represent operating a fuelcell with poor efficiency. Instead, the waste heat can be generated dueto operating a fuel cell at an increased power density. Part ofimproving the power density of a fuel cell can include operating thefuel cell at a sufficiently high current density. In various aspects,the current density generated by the fuel cell can be at least about 150mA/cm², such as at least about 160 mA/cm², or at least about 170 mA/cm²,or at least about 180 mA/cm², or at least about 190 mA/cm², or at leastabout 200 mA/cm², or at least about 225 mA/cm², or at least about 250mA/cm². Additionally or alternately, the current density generated bythe fuel cell can be about 500 mA/cm² or less, such as 450 mA/cm², orless, or 400 mA/cm², or less or 350 mA/cm², or less or 300 mA/cm² orless.

In various aspects, to allow a fuel cell to be operated with increasedpower generation and increased generation of waste heat, an effectiveamount of an endothermic reaction (such as a reforming reaction) can beperformed. Alternatively, other endothermic reactions unrelated to anodeoperations can be used to utilize the waste heat by interspersing“plates” or stages into the fuel cell array in thermal communication butnot in fluid communication with either anodes or cathodes. The effectiveamount of the endothermic reaction can be performed in an associatedreforming stage, an integrated reforming stage, an integrated stackelement for performing an endothermic reaction, or a combinationthereof. The effective amount of the endothermic reaction can correspondto an amount sufficient to reduce the temperature rise from the fuelcell inlet to the fuel cell outlet to about 100° C. or less, such asabout 90° C. or less, or about 80° C. or less, or about 70° C. or less,or about 60° C. or less, or about 50° C. or less, or about 40° C. orless, or about 30° C. or less. Additionally or alternately, theeffective amount of the endothermic reaction can correspond to an amountsufficient to cause a temperature decrease from the fuel cell inlet tothe fuel cell outlet of about 100° C. or less, such as about 90° C. orless, or about 80° C. or less, or about 70° C. or less, or about 60° C.or less, or about 50° C. or less, or about 40° C. or less, or about 30°C. or less, or about 20° C. or less, or about 10° C. or less. Atemperature decrease from the fuel cell inlet to the fuel cell outletcan occur when the effective amount of the endothermic reaction exceedsthe waste heat generated. Additionally or alternately, this cancorrespond to having the endothermic reaction(s) (such as a combinationof reforming and another endothermic reaction) consume at least about40% of the waste heat generated by the fuel cell, such as consuming atleast about 50% of the waste heat, or at least about 60% of the wasteheat, or at least about 75% of the waste heat. Further additionally oralternately, the endothermic reaction(s) can consume about 95% of thewaste heat or less, such as about 90% of the waste heat or less, orabout 85% of the waste heat or less.

DEFINITIONS

Syngas: In this description, syngas is defined as mixture of H₂ and COin any ratio. Optionally, H₂O and/or CO₂ may be present in the syngas.Optionally, inert compounds (such as nitrogen) and residual reformablefuel compounds may be present in the syngas. If components other than H₂and CO are present in the syngas, the combined volume percentage of H₂and CO in the syngas can be at least 25 vol % relative to the totalvolume of the syngas, such as at least 40 vol %, or at least 50 vol %,or at least 60 vol %. Additionally or alternately, the combined volumepercentage of H₂ and CO in the syngas can be 100 vol % or less, such as95 vol % or less or 90 vol % or less.

Reformable fuel: A reformable fuel is defined as a fuel that containscarbon-hydrogen bonds that can be reformed to generate H₂. Hydrocarbonsare examples of reformable fuels, as are other hydrocarbonaceouscompounds such as alcohols. Although CO and H₂O can participate in awater gas shift reaction to form hydrogen, CO is not considered areformable fuel under this definition.

Reformable hydrogen content: The reformable hydrogen content of a fuelis defined as the number of H₂ molecules that can be derived from a fuelby reforming the fuel and then driving the water gas shift reaction tocompletion to maximize H₂ production. It is noted that H₂ by definitionhas a reformable hydrogen content of 1, although H₂ itself is notdefined as a reformable fuel herein. Similarly, CO has a reformablehydrogen content of 1. Although CO is not strictly reformable, drivingthe water gas shift reaction to completion will result in exchange of aCO for an H₂. As examples of reformable hydrogen content for reformablefuels, the reformable hydrogen content of methane is 4 H₂ moleculeswhile the reformable hydrogen content of ethane is 7 H₂ molecules. Moregenerally, if a fuel has the composition CxHyOz, then the reformablehydrogen content of the fuel at 100% reforming and water-gas shift isn(H₂ max reforming)=2x+y/2−z. Based on this definition, fuel utilizationwithin a cell can then be expressed as n(H₂ ox)/n(H₂ max reforming) Ofcourse, the reformable hydrogen content of a mixture of components canbe determined based on the reformable hydrogen content of the individualcomponents. The reformable hydrogen content of compounds that containother heteroatoms, such as oxygen, sulfur or nitrogen, can also becalculated in a similar manner.

Oxidation Reaction: In this discussion, the oxidation reaction withinthe anode of a fuel cell is defined as the reaction corresponding tooxidation of H₂ by reaction with CO₃ ²⁻ to form H₂O and CO₂. It is notedthat the reforming reaction within the anode, where a compoundcontaining a carbon-hydrogen bond is converted into H₂ and CO or CO₂, isexcluded from this definition of the oxidation reaction in the anode.The water-gas shift reaction is similarly outside of this definition ofthe oxidation reaction. It is further noted that references to acombustion reaction are defined as references to reactions where H₂ or acompound containing carbon-hydrogen bond(s) are reacted with O₂ to formH₂O and carbon oxides in a non-electrochemical burner, such as thecombustion zone of a combustion-powered generator.

Aspects of the invention can adjust anode fuel parameters to achieve adesired operating range for the fuel cell. Anode fuel parameters can becharacterized directly, and/or in relation to other fuel cell processesin the form of one or more ratios. For example, the anode fuelparameters can be controlled to achieve one or more ratios including afuel utilization, a fuel cell heating value utilization, a fuel surplusratio, a reformable fuel surplus ratio, a reformable hydrogen contentfuel ratio, and combinations thereof.

Fuel utilization: Fuel utilization is an option for characterizingoperation of the anode based on the amount of oxidized fuel relative tothe reformable hydrogen content of an input stream can be used to definea fuel utilization for a fuel cell. In this discussion, “fuelutilization” is defined as the ratio of the amount of hydrogen oxidizedin the anode for production of electricity (as described above) versusthe reformable hydrogen content of the anode input (including anyassociated reforming stages). Reformable hydrogen content has beendefined above as the number of H₂ molecules that can be derived from afuel by reforming the fuel and then driving the water gas shift reactionto completion to maximize H₂ production. For example, each methaneintroduced into an anode and exposed to steam reforming conditionsresults in generation of the equivalent of 4 H₂ molecules at maxproduction. (Depending on the reforming and/or anode conditions, thereforming product can correspond to a non-water gas shifted product,where one or more of the H₂ molecules is present instead in the form ofa CO molecule.) Thus, methane is defined as having a reformable hydrogencontent of 4 H₂ molecules. As another example, under this definitionethane has a reformable hydrogen content of 7 H₂ molecules.

The utilization of fuel in the anode can also be characterized bydefining a heating value utilization based on a ratio of the LowerHeating Value of hydrogen oxidized in the anode due to the fuel cellanode reaction relative to the Lower Heating Value of all fuel deliveredto the anode and/or a reforming stage associated with the anode. The“fuel cell heating value utilization” as used herein can be computedusing the flow rates and Lower Heating Value (LHV) of the fuelcomponents entering and leaving the fuel cell anode. As such, fuel cellheating value utilization can be computed as(LHV(anode_in)−LHV(anode_out))/LHV(anode_in), where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In this definition, the LHV of a stream or flow may be computed as a sumof values for each fuel component in the input and/or output stream. Thecontribution of each fuel component to the sum can correspond to thefuel component's flow rate (e.g., mol/hr) multiplied by the fuelcomponent's LHV (e.g., joules/mol).

Lower Heating Value: The lower heating value is defined as the enthalpyof combustion of a fuel component to vapor phase, fully oxidizedproducts (i.e., vapor phase CO₂ and H₂O product). For example, any CO₂present in an anode input stream does not contribute to the fuel contentof the anode input, since CO₂ is already fully oxidized. For thisdefinition, the amount of oxidation occurring in the anode due to theanode fuel cell reaction is defined as oxidation of H₂ in the anode aspart of the electrochemical reaction in the anode, as defined above.

It is noted that, for the special case where the only fuel in the anodeinput flow is H₂, the only reaction involving a fuel component that cantake place in the anode represents the conversion of H₂ into H₂O. Inthis special case, the fuel utilization simplifies to (H₂-rate-in minusH₂-rate-out)/H₂-rate-in. In such a case, H₂ would be the only fuelcomponent, and so the H₂ LHV would cancel out of the equation. In themore general case, the anode feed may contain, for example, CH₄, H₂, andCO in various amounts. Because these species can typically be present indifferent amounts in the anode outlet, the summation as described abovecan be needed to determine the fuel utilization.

Alternatively or in addition to fuel utilization, the utilization forother reactants in the fuel cell can be characterized. For example, theoperation of a fuel cell can additionally or alternately becharacterized with regard to “CO₂ utilization” and/or “oxidant”utilization. The values for CO₂ utilization and/or oxidant utilizationcan be specified in a similar manner.

Fuel surplus ratio: Still another way to characterize the reactions in amolten carbonate fuel cell is by defining a utilization based on a ratioof the Lower Heating Value of all fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. This quantity will be referred to as a fuel surplus ratio. Assuch the fuel surplus ratio can be computed as (LHV(anode_in)/(LHV(anode_in)−LHV(anode_out)) where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In various aspects of the invention, a molten carbonate fuel cell can beoperated to have a fuel surplus ratio of at least about 1.0, such as atleast about 1.5, or at least about 2.0, or at least about 2.5, or atleast about 3.0, or at least about 4.0. Additionally or alternately, thefuel surplus ratio can be about 25.0 or less.

It is noted that not all of the reformable fuel in the input stream forthe anode may be reformed. Preferably, at least about 90% of thereformable fuel in the input stream to the anode (and/or into anassociated reforming stage) can be reformed prior to exiting the anode,such as at least about 95% or at least about 98%. In some alternativeaspects, the amount of reformable fuel that is reformed can be fromabout 75% to about 90%, such as at least about 80%.

The above definition for fuel surplus ratio provides a method forcharacterizing the amount of reforming occurring within the anode and/orreforming stage(s) associated with a fuel cell relative to the amount offuel consumed in the fuel cell anode for generation of electric power.

Optionally, the fuel surplus ratio can be modified to account forsituations where fuel is recycled from the anode output to the anodeinput. When fuel (such as H₂, CO, and/or unreformed or partiallyreformed hydrocarbons) is recycled from anode output to anode input,such recycled fuel components do not represent a surplus amount ofreformable or reformed fuel that can be used for other purposes.Instead, such recycled fuel components merely indicate a desire toreduce fuel utilization in a fuel cell.

Reformable fuel surplus ratio: Calculating a reformable fuel surplusratio is one option to account for such recycled fuel components is tonarrow the definition of surplus fuel, so that only the LHV ofreformable fuels is included in the input stream to the anode. As usedherein the “reformable fuel surplus ratio” is defined as the LowerHeating Value of reformable fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. Under the definition for reformable fuel surplus ratio, theLHV of any H₂ or CO in the anode input is excluded. Such an LHV ofreformable fuel can still be measured by characterizing the actualcomposition entering a fuel cell anode, so no distinction betweenrecycled components and fresh components needs to be made. Although somenon-reformed or partially reformed fuel may also be recycled, in mostaspects the majority of the fuel recycled to the anode can correspond toreformed products such as H₂ or CO. Expressed mathematically, thereformable fuel surplus ratio (R_(RFS))=LHV_(RF)/LHV_(OH), whereLHV_(RF) is the Lower Heating Value (LHV) of the reformable fuel andLHV_(OH) is the Lower Heating Value (LHV) of the hydrogen oxidized inthe anode. The LHV of the hydrogen oxidized in the anode may becalculated by subtracting the LHV of the anode outlet stream from theLHV of the anode inlet stream (e.g., LHV(anode_in)−LHV(anode_out)). Invarious aspects of the invention, a molten carbonate fuel cell can beoperated to have a reformable fuel surplus ratio of at least about 0.25,such as at least about 0.5, or at least about 1.0, or at least about1.5, or at least about 2.0, or at least about 2.5, or at least about3.0, or at least about 4.0. Additionally or alternately, the reformablefuel surplus ratio can be about 25.0 or less. It is noted that thisnarrower definition based on the amount of reformable fuel delivered tothe anode relative to the amount of oxidation in the anode candistinguish between two types of fuel cell operation methods that havelow fuel utilization. Some fuel cells achieve low fuel utilization byrecycling a substantial portion of the anode output back to the anodeinput. This recycle can allow any hydrogen in the anode input to be usedagain as an input to the anode. This can reduce the amount of reforming,as even though the fuel utilization is low for a single pass through thefuel cell, at least a portion of the unused fuel is recycled for use ina later pass. Thus, fuel cells with a wide variety of fuel utilizationvalues may have the same ratio of reformable fuel delivered to the anodereforming stage(s) versus hydrogen oxidized in the anode reaction. Inorder to change the ratio of reformable fuel delivered to the anodereforming stages relative to the amount of oxidation in the anode,either an anode feed with a native content of non-reformable fuel needsto be identified, or unused fuel in the anode output needs to bewithdrawn for other uses, or both.

Reformable hydrogen surplus ratio: Still another option forcharacterizing the operation of a fuel cell is based on a “reformablehydrogen surplus ratio.” The reformable fuel surplus ratio defined aboveis defined based on the lower heating value of reformable fuelcomponents. The reformable hydrogen surplus ratio is defined as thereformable hydrogen content of reformable fuel delivered to the anodeand/or a reforming stage associated with the anode relative to thehydrogen reacted in the anode due to the fuel cell anode reaction. Assuch, the “reformable hydrogen surplus ratio” can be computed as(RFC(reformable_anode_in)/(RFC(reformable_anode_in)−RFC(anode_out)),where RFC(reformable_anode_in) refers to the reformable hydrogen contentof reformable fuels in the anode inlet streams or flows, while RFC(anode_out) refers to the reformable hydrogen content of the fuelcomponents (such as H₂, CH₄, and/or CO) in the anode inlet and outletstreams or flows. The RFC can be expressed in moles/s, moles/hr, orsimilar. An example of a method for operating a fuel cell with a largeratio of reformable fuel delivered to the anode reforming stage(s)versus amount of oxidation in the anode can be a method where excessreforming is performed in order to balance the generation andconsumption of heat in the fuel cell. Reforming a reformable fuel toform H₂ and CO is an endothermic process. This endothermic reaction canbe countered by the generation of electrical current in the fuel cell,which can also produce excess heat corresponding (roughly) to thedifference between the amount of heat generated by the anode oxidationreaction and the carbonate formation reaction and the energy that exitsthe fuel cell in the form of electric current. The excess heat per moleof hydrogen involved in the anode oxidation reaction/carbonate formationreaction can be greater than the heat absorbed to generate a mole ofhydrogen by reforming. As a result, a fuel cell operated underconventional conditions can exhibit a temperature increase from inlet tooutlet. Instead of this type of conventional operation, the amount offuel reformed in the reforming stages associated with the anode can beincreased. For example, additional fuel can be reformed so that the heatgenerated by the exothermic fuel cell reactions can be (roughly)balanced by the heat consumed in reforming, or even the heat consumed byreforming can be beyond the excess heat generated by the fuel oxidation,resulting in a temperature drop across the fuel cell. This can result ina substantial excess of hydrogen relative to the amount needed forelectrical power generation. As one example, a feed to the anode inletof a fuel cell or an associated reforming stage can be substantiallycomposed of reformable fuel, such as a substantially pure methane feed.During conventional operation for electric power generation using such afuel, a molten carbonate fuel cell can be operated with a fuelutilization of about 75%. This means that about 75% (or ¾) of the fuelcontent delivered to the anode is used to form hydrogen that is thenreacted in the anode with carbonate ions to form H₂O and CO₂. Inconventional operation, the remaining about 25% of the fuel content canbe reformed to H₂ within the fuel cell (or can pass through the fuelcell unreacted for any CO or H₂ in the fuel), and then combusted outsideof the fuel cell to form H₂O and CO₂ to provide heat for the cathodeinlet to the fuel cell. The reformable hydrogen surplus ratio in thissituation can be 4/(4−1)=4/3.

Electrical efficiency: As used herein, the term “electrical efficiency”(“EE”) is defined as the electrochemical power produced by the fuel celldivided by the rate of Lower Heating Value (“LHV”) of fuel input to thefuel cell. The fuel inputs to the fuel cell includes both fuel deliveredto the anode as well as any fuel used to maintain the temperature of thefuel cell, such as fuel delivered to a burner associated with a fuelcell. In this description, the power produced by the fuel may bedescribed in terms of LHV(el) fuel rate.

Electrochemical power: As used herein, the term “electrochemical power”or LHV(el) is the power generated by the circuit connecting the cathodeto the anode in the fuel cell and the transfer of carbonate ions acrossthe fuel cell's electrolyte. Electrochemical power excludes powerproduced or consumed by equipment upstream or downstream from the fuelcell. For example, electricity produced from heat in a fuel cell exhauststream is not considered part of the electrochemical power. Similarly,power generated by a gas turbine or other equipment upstream of the fuelcell is not part of the electrochemical power generated. The“electrochemical power” does not take electrical power consumed duringoperation of the fuel cell into account, or any loss incurred byconversion of the direct current to alternating current. In other words,electrical power used to supply the fuel cell operation or otherwiseoperate the fuel cell is not subtracted from the direct current powerproduced by the fuel cell. As used herein, the power density is thecurrent density multiplied by voltage. As used herein, the total fuelcell power is the power density multiplied by the fuel cell area.

Fuel inputs: As used herein, the term “anode fuel input,” designated asLHV(anode_in), is the amount of fuel within the anode inlet stream. Theterm “fuel input”, designated as LHV(in), is the total amount of fueldelivered to the fuel cell, including both the amount of fuel within theanode inlet stream and the amount of fuel used to maintain thetemperature of the fuel cell. The fuel may include both reformable andnonreformable fuels, based on the definition of a reformable fuelprovided herein. Fuel input is not the same as fuel utilization.

Total fuel cell efficiency: As used herein, the term “total fuel cellefficiency” (“TFCE”) is defined as: the electrochemical power generatedby the fuel cell, plus the rate of LHV of syngas produced by the fuelcell, divided by the rate of LHV of fuel input to the anode. In otherwords, TFCE=(LHV(el)+LHV(sg net))/LHV(anode_in), where LHV(anode_in)refers to rate at which the LHV of the fuel components (such as H₂, CH₄,and/or CO) delivered to the anode and LHV(sg net) refers to a rate atwhich syngas (H₂, CO) is produced in the anode, which is the differencebetween syngas input to the anode and syngas output from the anode.LHV(el) describes the electrochemical power generation of the fuel cell.The total fuel cell efficiency excludes heat generated by the fuel cellthat is put to beneficial use outside of the fuel cell. In operation,heat generated by the fuel cell may be put to beneficial use bydownstream equipment. For example, the heat may be used to generateadditional electricity or to heat water. These uses, when they occurapart from the fuel cell, are not part of the total fuel cellefficiency, as the term is used in this application. The total fuel cellefficiency is for the fuel cell operation only, and does not includepower production, or consumption, upstream, or downstream, of the fuelcell.

Chemical efficiency: As used herein, the term “chemical efficiency”, isdefined as the lower heating value of H₂ and CO in the anode exhaust ofthe fuel cell, or LHV(sg out), divided by the fuel input, or LHV(in).

Neither the electrical efficiency nor the total system efficiency takesthe efficiency of upstream or downstream processes into consideration.For example, it may be advantageous to use turbine exhaust as a sourceof CO₂ for the fuel cell cathode. In this arrangement, the efficiency ofthe turbine is not considered as part of the electrical efficiency orthe total fuel cell efficiency calculation. Similarly, outputs from thefuel cell may be recycled as inputs to the fuel cell. A recycle loop isnot considered when calculating electrical efficiency or the total fuelcell efficiency in single pass mode.

Syngas produced: As used herein, the term “syngas produced” is thedifference between syngas input to the anode and syngas output from theanode. Syngas may be used as an input, or fuel, for the anode, at leastin part. For example, a system may include an anode recycle loop thatreturns syngas from the anode exhaust to the anode inlet where it issupplemented with natural gas or other suitable fuel. Syngas producedLHV (sg net)=(LHV(sg out)−LHV(sg in)), where LHV(sg in) and LHV(sg out)refer to the LHV of the syngas in the anode inlet and syngas in theanode outlet streams or flows, respectively. It is noted that at least aportion of the syngas produced by the reforming reactions within ananode can typically be utilized in the anode to produce electricity. Thehydrogen utilized to produce electricity is not included in thedefinition of “syngas produced” because it does not exit the anode. Asused herein, the term “syngas ratio” is the LHV of the net syngasproduced divided by the LHV of the fuel input to the anode or LHV (sgnet)/LHV(anode in). Molar flow rates of syngas and fuel can be usedinstead of LHV to express a molar-based syngas ratio and a molar-basedsyngas produced.

Steam to carbon ratio (S/C): As used herein, the steam to carbon ratio(S/C) is the molar ratio of steam in a flow to reformable carbon in theflow. Carbon in the form of CO and CO₂ are not included as reformablecarbon in this definition. The steam to carbon ratio can be measuredand/or controlled at different points in the system. For example, thecomposition of an anode inlet stream can be manipulated to achieve a S/Cthat is suitable for reforming in the anode. The S/C can be given as themolar flow rate of H₂O divided by the product of the molar flow rate offuel multiplied by the number of carbon atoms in the fuel, e.g. one formethane. Thus, S/C=f_(H20)/(f_(CH4) X #C), where f_(H20) is the molarflow rate of water, where f_(CH4) is the molar flow rate of methane (orother fuel) and #C is the number of carbons in the fuel.

EGR ratio: Aspects of the invention can use a turbine in partnershipwith a fuel cell. The combined fuel cell and turbine system may includeexhaust gas recycle (“EGR”). In an EGR system, at least a portion of theexhaust gas generated by the turbine can be sent to a heat recoverygenerator. Another portion of the exhaust gas can be sent to the fuelcell. The EGR ratio describes the amount of exhaust gas routed to thefuel cell versus the total exhaust gas routed to either the fuel cell orheat recovery generator. As used herein, the “EGR ratio” is the flowrate for the fuel cell bound portion of the exhaust gas divided by thecombined flow rate for the fuel cell bound portion and the recoverybound portion, which is sent to the heat recovery generator.

In various aspects of the invention, a molten carbonate fuel cell (MCFC)can be used to facilitate separation of CO₂ from a CO₂-containing streamwhile also generating additional electrical power. The CO₂ separationcan be further enhanced by taking advantage of synergies with thecombustion-based power generator that can provide at least a portion ofthe input feed to the cathode portion of the fuel cell.

Fuel Cell and Fuel Cell Components: In this discussion, a fuel cell cancorrespond to a single cell, with an anode and a cathode separated by anelectrolyte. The anode and cathode can receive input gas flows tofacilitate the respective anode and cathode reactions for transportingcharge across the electrolyte and generating electricity. A fuel cellstack can represent a plurality of cells in an integrated unit. Althougha fuel cell stack can include multiple fuel cells, the fuel cells cantypically be connected in parallel and can function (approximately) asif they collectively represented a single fuel cell of a larger size.When an input flow is delivered to the anode or cathode of a fuel cellstack, the fuel stack can include flow channels for dividing the inputflow between each of the cells in the stack and flow channels forcombining the output flows from the individual cells. In thisdiscussion, a fuel cell array can be used to refer to a plurality offuel cells (such as a plurality of fuel cell stacks) that are arrangedin series, in parallel, or in any other convenient manner (e.g., in acombination of series and parallel). A fuel cell array can include oneor more stages of fuel cells and/or fuel cell stacks, where theanode/cathode output from a first stage may serve as the anode/cathodeinput for a second stage. It is noted that the anodes in a fuel cellarray do not have to be connected in the same way as the cathodes in thearray. For convenience, the input to the first anode stage of a fuelcell array may be referred to as the anode input for the array, and theinput to the first cathode stage of the fuel cell array may be referredto as the cathode input to the array. Similarly, the output from thefinal anode/cathode stage may be referred to as the anode/cathode outputfrom the array.

It should be understood that reference to use of a fuel cell hereintypically denotes a “fuel cell stack” composed of individual fuel cells,and more generally refers to use of one or more fuel cell stacks influid communication. Individual fuel cell elements (plates) cantypically be “stacked” together in a rectangular array called a “fuelcell stack”. This fuel cell stack can typically take a feed stream anddistribute reactants among all of the individual fuel cell elements andcan then collect the products from each of these elements. When viewedas a unit, the fuel cell stack in operation can be taken as a whole eventhough composed of many (often tens or hundreds) of individual fuel cellelements. These individual fuel cell elements can typically have similarvoltages (as the reactant and product concentrations are similar), andthe total power output can result from the summation of all of theelectrical currents in all of the cell elements, when the elements areelectrically connected in series. Stacks can also be arranged in aseries arrangement to produce high voltages. A parallel arrangement canboost the current. If a sufficiently large volume fuel cell stack isavailable to process a given exhaust flow, the systems and methodsdescribed herein can be used with a single molten carbonate fuel cellstack. In other aspects of the invention, a plurality of fuel cellstacks may be desirable or needed for a variety of reasons.

For the purposes of this invention, unless otherwise specified, the term“fuel cell” should be understood to also refer to and/or is defined asincluding a reference to a fuel cell stack composed of set of one ormore individual fuel cell elements for which there is a single input andoutput, as that is the manner in which fuel cells are typically employedin practice. Similarly, the term fuel cells (plural), unless otherwisespecified, should be understood to also refer to and/or is defined asincluding a plurality of separate fuel cell stacks. In other words, allreferences within this document, unless specifically noted, can referinterchangeably to the operation of a fuel cell stack as a “fuel cell”.For example, the volume of exhaust generated by a commercial scalecombustion generator may be too large for processing by a fuel cell(i.e., a single stack) of conventional size. In order to process thefull exhaust, a plurality of fuel cells (i.e., two or more separate fuelcells or fuel cell stacks) can be arranged in parallel, so that eachfuel cell can process (roughly) an equal portion of the combustionexhaust. Although multiple fuel cells can be used, each fuel cell cantypically be operated in a generally similar manner, given its (roughly)equal portion of the combustion exhaust.

“Internal reforming” and “external reforming”: A fuel cell or fuel cellstack may include one or more internal reforming sections. As usedherein, the term “internal reforming” refers to fuel reforming occurringwithin the body of a fuel cell, a fuel cell stack, or otherwise within afuel cell assembly. External reforming, which is often used inconjunction with a fuel cell, occurs in a separate piece of equipmentthat is located outside of the fuel cell stack. In other words, the bodyof the external reformer is not in direct physical contact with the bodyof a fuel cell or fuel cell stack. In a typical set up, the output fromthe external reformer can be fed to the anode inlet of a fuel cell.Unless otherwise noted specifically, the reforming described within thisapplication is internal reforming.

Internal reforming may occur within a fuel cell anode. Internalreforming can additionally or alternately occur within an internalreforming element integrated within a fuel cell assembly. The integratedreforming element may be located between fuel cell elements within afuel cell stack. In other words, one of the trays in the stack can be areforming section instead of a fuel cell element. In one aspect, theflow arrangement within a fuel cell stack directs fuel to the internalreforming elements and then into the anode portion of the fuel cells.Thus, from a flow perspective, the internal reforming elements and fuelcell elements can be arranged in series within the fuel cell stack. Asused herein, the term “anode reforming” is fuel reforming that occurswithin an anode. As used herein, the term “internal reforming” isreforming that occurs within an integrated reforming element and not inan anode section.

In some aspects, a reforming stage that is internal to a fuel cellassembly can be considered to be associated with the anode(s) in thefuel cell assembly. In some alternative aspects, for a reforming stagein a fuel cell stack that can be associated with an anode (such asassociated with multiple anodes), a flow path can be available so thatthe output flow from the reforming stage is passed into at least oneanode. This can correspond to having an initial section of a fuel cellplate that is not in contact with the electrolyte and instead servesjust as a reforming catalyst. Another option for an associated reformingstage can be to have a separate integrated reforming stage as one of theelements in a fuel cell stack, where the output from the integratedreforming stage is returned to the input side of one or more of the fuelcells in the fuel cell stack.

From a heat integration standpoint, a characteristic height in a fuelcell stack can be the height of an individual fuel cell stack element.It is noted that the separate reforming stage or a separate endothermicreaction stage could have a different height in the stack than a fuelcell. In such a scenario, the height of a fuel cell element can be usedas the characteristic height. In some aspects, an integrated endothermicreaction stage can be defined as a stage that is heat integrated withone or more fuel cells, so that the integrated endothermic reactionstage can use the heat from the fuel cells as a heat source forreforming. Such an integrated endothermic reaction stage can be definedas being positioned less than 5 times the height of a stack element fromany fuel cells providing heat to the integrated stage. For example, anintegrated endothermic reaction stage (such as a reforming stage) can bepositioned less than 5 times the height of a stack element from any fuelcells that are heat integrated, such as less than 3 times the height ofa stack element. In this discussion, an integrated reforming stage orintegrated endothermic reaction stage that represents an adjacent stackelement to a fuel cell element can be defined as being about one stackelement height or less away from the adjacent fuel cell element.

In some aspects, a separate reforming stage that is heat integrated witha fuel cell element can also correspond to a reforming stage that isassociated with the fuel cell element. In such aspects, an integratedfuel cell element can provide at least a portion of the heat to theassociated reforming stage, and the associated reforming stage canprovide at least a portion of the reforming stage output to theintegrated fuel cell as a fuel stream. In other aspects, a separatereforming stage can be integrated with a fuel cell for heat transferwithout being associated with the fuel cell. In this type of situation,the separate reforming stage can receive heat from the fuel cell, butthe output of the reforming stage is not used as an input to the fuelcell. Instead, the output of such a reforming stage can be used foranother purpose, such as directly adding the output to the anode exhauststream, or for forming a separate output stream from the fuel cellassembly.

More generally, a separate stack element in a fuel cell stack can beused to perform any convenient type of endothermic reaction that cantake advantage of the waste heat provided by integrated fuel cell stackelements. Instead of plates suitable for performing a reforming reactionon a hydrocarbon fuel stream, a separate stack element can have platessuitable for catalyzing another type of endothermic reaction. A manifoldor other arrangement of inlet conduits in the fuel cell stack can beused to provide an appropriate input flow to each stack element. Asimilar manifold or other arrangement of outlet conduits can also beused to withdraw the output flows from each stack element. Optionally,the output flows from a endothermic reaction stage in a stack can bewithdrawn from the fuel cell stack without having the output flow passthrough a fuel cell anode. In such an optional aspect, the products ofthe exothermic reaction will therefore exit from the fuel cell stackwithout passing through a fuel cell anode. Examples of other types ofendothermic reactions that can be performed in stack elements in a fuelcell stack include ethanol dehydration to form ethylene and ethanecracking.

Recycle: As defined herein, recycle of a portion of a fuel cell output(such as an anode exhaust or a stream separated or withdrawn from ananode exhaust) to a fuel cell inlet can correspond to a direct orindirect recycle stream. A direct recycle of a stream to a fuel cellinlet is defined as recycle of the stream without passing through anintermediate process, while an indirect recycle involves recycle afterpassing a stream through one or more intermediate processes. Forexample, if the anode exhaust is passed through a CO₂ separation stageprior to recycle, this is considered an indirect recycle of the anodeexhaust. If a portion of the anode exhaust, such as an H₂ streamwithdrawn from the anode exhaust, is passed into a gasifier forconverting coal into a fuel suitable for introduction into the fuelcell, then that is also considered an indirect recycle.

Anode Inputs and Outputs

In various aspects of the invention, the MCFC array can be fed by a fuelreceived at the anode inlet that comprises, for example, both hydrogenand a hydrocarbon such as methane (or alternatively a hydrocarbonaceousor hydrocarbon-like compound that may contain heteroatoms different fromC and H). Most of the methane (or other hydrocarbonaceous orhydrocarbon-like compound) fed to the anode can typically be freshmethane. In this description, a fresh fuel such as fresh methane refersto a fuel that is not recycled from another fuel cell process. Forexample, methane recycled from the anode outlet stream back to the anodeinlet may not be considered “fresh” methane, and can instead bedescribed as reclaimed methane. The fuel source used can be shared withother components, such as a turbine that uses a portion of the fuelsource to provide a CO₂-containing stream for the cathode input. Thefuel source input can include water in a proportion to the fuelappropriate for reforming the hydrocarbon (or hydrocarbon-like) compoundin the reforming section that generates hydrogen. For example, ifmethane is the fuel input for reforming to generate H₂, the molar ratioof water to fuel can be from about one to one to about ten to one, suchas at least about two to one. A ratio of four to one or greater istypical for external reforming, but lower values can be typical forinternal reforming. To the degree that H₂ is a portion of the fuelsource, in some optional aspects no additional water may be needed inthe fuel, as the oxidation of H₂ at the anode can tend to produce H₂Othat can be used for reforming the fuel. The fuel source can alsooptionally contain components incidental to the fuel source (e.g., anatural gas feed can contain some content of CO₂ as an additionalcomponent). For example, a natural gas feed can contain CO₂, N₂, and/orother inert (noble) gases as additional components. Optionally, in someaspects the fuel source may also contain CO, such as CO from a recycledportion of the anode exhaust. An additional or alternate potentialsource for CO in the fuel into a fuel cell assembly can be CO generatedby steam reforming of a hydrocarbon fuel performed on the fuel prior toentering the fuel cell assembly.

More generally, a variety of types of fuel streams may be suitable foruse as an input stream for the anode of a molten carbonate fuel cell.Some fuel streams can correspond to streams containing hydrocarbonsand/or hydrocarbon-like compounds that may also include heteroatomsdifferent from C and H. In this discussion, unless otherwise specified,a reference to a fuel stream containing hydrocarbons for an MCFC anodeis defined to include fuel streams containing such hydrocarbon-likecompounds.

Examples of hydrocarbon (including hydrocarbon-like) fuel streamsinclude natural gas, streams containing C1-C4 carbon compounds (such asmethane or ethane), and streams containing heavier C5+ hydrocarbons(including hydrocarbon-like compounds), as well as combinations thereof.Still other additional or alternate examples of potential fuel streamsfor use in an anode input can include biogas-type streams, such asmethane produced from natural (biological) decomposition of organicmaterial.

In some aspects, a molten carbonate fuel cell can be used to process aninput fuel stream, such as a natural gas and/or hydrocarbon stream, witha low energy content due to the presence of diluent compounds. Forexample, some sources of methane and/or natural gas are sources that caninclude substantial amounts of either CO₂ or other inert molecules, suchas nitrogen, argon, or helium. Due to the presence of elevated amountsof CO₂ and/or inerts, the energy content of a fuel stream based on thesource can be reduced. Using a low energy content fuel for a combustionreaction (such as for powering a combustion-powered turbine) can posedifficulties. However, a molten carbonate fuel cell can generate powerbased on a low energy content fuel source with a reduced or minimalimpact on the efficiency of the fuel cell. The presence of additionalgas volume can require additional heat for raising the temperature ofthe fuel to the temperature for reforming and/or the anode reaction.Additionally, due to the equilibrium nature of the water gas shiftreaction within a fuel cell anode, the presence of additional CO₂ canhave an impact on the relative amounts of H₂ and CO present in the anodeoutput. However, the inert compounds otherwise can have only a minimaldirect impact on the reforming and anode reactions. The amount of CO₂and/or inert compounds in a fuel stream for a molten carbonate fuelcell, when present, can be at least about 1 vol %, such as at leastabout 2 vol %, or at least about 5 vol %, or at least about 10 vol %, orat least about 15 vol %, or at least about 20 vol %, or at least about25 vol %, or at least about 30 vol %, or at least about 35 vol %, or atleast about 40 vol %, or at least about 45 vol %, or at least about 50vol %, or at least about 75 vol %. Additionally or alternately, theamount of CO₂ and/or inert compounds in a fuel stream for a moltencarbonate fuel cell can be about 90 vol % or less, such as about 75 vol% or less, or about 60 vol % or less, or about 50 vol % or less, orabout 40 vol % or less, or about 35 vol % or less.

Yet other examples of potential sources for an anode input stream cancorrespond to refinery and/or other industrial process output streams.For example, coking is a common process in many refineries forconverting heavier compounds to lower boiling ranges. Coking typicallyproduces an off-gas containing a variety of compounds that are gases atroom temperature, including CO and various C1-C4 hydrocarbons. Thisoff-gas can be used as at least a portion of an anode input stream.Other refinery off-gas streams can additionally or alternately besuitable for inclusion in an anode input stream, such as light ends(C1-C4) generated during cracking or other refinery processes. Stillother suitable refinery streams can additionally or alternately includerefinery streams containing CO or CO₂ that also contain H₂ and/orreformable fuel compounds.

Still other potential sources for an anode input can additionally oralternately include streams with increased water content. For example,an ethanol output stream from an ethanol plant (or another type offermentation process) can include a substantial portion of H₂O prior tofinal distillation. Such H₂O can typically cause only minimal impact onthe operation of a fuel cell. Thus, a fermentation mixture of alcohol(or other fermentation product) and water can be used as at least aportion of an anode input stream.

Biogas, or digester gas, is another additional or alternate potentialsource for an anode input. Biogas may primarily comprise methane and CO₂and is typically produced by the breakdown or digestion of organicmatter. Anaerobic bacteria may be used to digest the organic matter andproduce the biogas. Impurities, such as sulfur-containing compounds, maybe removed from the biogas prior to use as an anode input.

The output stream from an MCFC anode can include H₂O, CO₂, CO, and H₂.Optionally, the anode output stream could also have unreacted fuel (suchas H₂ or CH₄) or inert compounds in the feed as additional outputcomponents. Instead of using this output stream as a fuel source toprovide heat for a reforming reaction or as a combustion fuel forheating the cell, one or more separations can be performed on the anodeoutput stream to separate the CO₂ from the components with potentialvalue as inputs to another process, such as H₂ or CO. The H₂ and/or COcan be used as a syngas for chemical synthesis, as a source of hydrogenfor chemical reaction, and/or as a fuel with reduced greenhouse gasemissions.

In various aspects, the composition of the output stream from the anodecan be impacted by several factors. Factors that can influence the anodeoutput composition can include the composition of the input stream tothe anode, the amount of current generated by the fuel cell, and/or thetemperature at the exit of the anode. The temperature of at the anodeexit can be relevant due to the equilibrium nature of the water gasshift reaction. In a typical anode, at least one of the plates formingthe wall of the anode can be suitable for catalyzing the water gas shiftreaction. As a result, if a) the composition of the anode input streamis known, b) the extent of reforming of reformable fuel in the anodeinput stream is known, and c) the amount of carbonate transported fromthe cathode to anode (corresponding to the amount of electrical currentgenerated) is known, the composition of the anode output can bedetermined based on the equilibrium constant for the water gas shiftreaction.

K_(eq)=[CO₂][H₂]/[CO][H₂O]

In the above equation, K_(eq) is the equilibrium constant for thereaction at a given temperature and pressure, and [X] is the partialpressure of component X. Based on the water gas shift reaction, it canbe noted that an increased CO₂ concentration in the anode input can tendto result in additional CO formation (at the expense of H₂) while anincreased H₂O concentration can tend to result in additional H₂formation (at the expense of CO).

To determine the composition at the anode output, the composition of theanode input can be used as a starting point. This composition can thenbe modified to reflect the extent of reforming of any reformable fuelsthat can occur within the anode. Such reforming can reduce thehydrocarbon content of the anode input in exchange for increasedhydrogen and CO₂. Next, based on the amount of electrical currentgenerated, the amount of H₂ in the anode input can be reduced inexchange for additional H₂O and CO₂. This composition can then beadjusted based on the equilibrium constant for the water gas shiftreaction to determine the exit concentrations for H₂, CO, CO₂, and H₂O.

Table 1 shows the anode exhaust composition at different fuelutilizations for a typical type of fuel. The anode exhaust compositioncan reflect the combined result of the anode reforming reaction, watergas shift reaction, and the anode oxidation reaction. The outputcomposition values in Table 1 were calculated by assuming an anode inputcomposition with an about 2 to 1 ratio of steam (H₂O) to carbon(reformable fuel). The reformable fuel was assumed to be methane, whichwas assumed to be 100% reformed to hydrogen. The initial CO₂ and H₂concentrations in the anode input were assumed to be negligible, whilethe input N₂ concentration was about 0.5%. The fuel utilization U_(f)(as defined herein) was allowed to vary from about 35% to about 70% asshown in the table. The exit temperature for the fuel cell anode wasassumed to be about 650° C. for purposes of determining the correctvalue for the equilibrium constant.

TABLE 1 Anode Exhaust Composition Uf % 35% 40% 45% 50% 55% 60% 65% 70%Anode Exhaust Composition H₂O %, wet 32.5% 34.1% 35.5% 36.7% 37.8% 38.9%39.8% 40.5% CO₂ %, wet 26.7% 29.4% 32.0% 34.5% 36.9% 39.3% 41.5% 43.8%H₂ %, wet 29.4% 26.0% 22.9% 20.0% 17.3% 14.8% 12.5% 10.4% CO %, wet10.8% 10.0% 9.2% 8.4% 7.5% 6.7% 5.8% 4.9% N₂ %, wet 0.5% 0.5% 0.5% 0.4%0.4% 0.4% 0.4% 0.4% CO₂ %, dry 39.6% 44.6% 49.6% 54.5% 59.4% 64.2% 69.0%73.7% H₂ %, dry 43.6% 39.4% 35.4% 31.5% 27.8% 24.2% 20.7% 17.5% CO %,dry 16.1% 15.2% 14.3% 13.2% 12.1% 10.9% 9.7% 8.2% N₂ %, dry 0.7% 0.7%0.7% 0.7% 0.7% 0.7% 0.7% 0.7% H₂/CO 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.1(H₂—CO₂)/ 0.07 −0.09 −0.22 −0.34 −0.44 −0.53 −0.61 −0.69 (CO + CO₂)

Table 1 shows anode output compositions for a particular set ofconditions and anode input composition. More generally, in variousaspects the anode output can include about 10 vol % to about 50 vol %H₂O. The amount of H₂O can vary greatly, as H₂O in the anode can beproduced by the anode oxidation reaction. If an excess of H₂O beyondwhat is needed for reforming is introduced into the anode, the excessH₂O can typically pass through largely unreacted, with the exception ofH₂O consumed (or generated) due to fuel reforming and the water gasshift reaction. The CO₂ concentration in the anode output can also varywidely, such as from about 20 vol % to about 50 vol % CO₂. The amount ofCO₂ can be influenced by both the amount of electrical current generatedas well as the amount of CO₂ in the anode input flow. The amount of H₂in the anode output can additionally or alternately be from about 10 vol% H₂ to about 50 vol % H₂, depending on the fuel utilization in theanode. At the anode output, the amount of CO can be from about 5 vol %to about 20 vol %. It is noted that the amount of CO relative to theamount of H₂ in the anode output for a given fuel cell can be determinedin part by the equilibrium constant for the water gas shift reaction atthe temperature and pressure present in the fuel cell. The anode outputcan further additionally or alternately include 5 vol % or less ofvarious other components, such as N₂, CH₄ (or other unreactedcarbon-containing fuels), and/or other components.

Optionally, one or more water gas shift reaction stages can be includedafter the anode output to convert CO and H₂O in the anode output intoCO₂ and H₂, if desired. The amount of H₂ present in the anode output canbe increased, for example, by using a water gas shift reactor at lowertemperature to convert H₂O and CO present in the anode output into H₂and CO₂. Alternatively, the temperature can be raised and the water-gasshift reaction can be reversed, producing more CO and H₂O from H₂ andCO₂. Water is an expected output of the reaction occurring at the anode,so the anode output can typically have an excess of H₂O relative to theamount of CO present in the anode output. Alternatively, H₂O can beadded to the stream after the anode exit but before the water gas shiftreaction. CO can be present in the anode output due to incomplete carbonconversion during reforming and/or due to the equilibrium balancingreactions between H₂O, CO, H₂, and CO₂ (i.e., the water-gas shiftequilibrium) under either reforming conditions or the conditions presentduring the anode reaction. A water gas shift reactor can be operatedunder conditions to drive the equilibrium further in the direction offorming CO₂ and H₂ at the expense of CO and H₂O. Higher temperatures cantend to favor the formation of CO and H₂O. Thus, one option foroperating the water gas shift reactor can be to expose the anode outputstream to a suitable catalyst, such as a catalyst including iron oxide,zinc oxide, copper on zinc oxide, or the like, at a suitabletemperature, e.g., between about 190° C. to about 210° C. Optionally,the water-gas shift reactor can include two stages for reducing the COconcentration in an anode output stream, with a first higher temperaturestage operated at a temperature from at least about 300° C. to about375° C. and a second lower temperature stage operated at a temperatureof about 225° C. or less, such as from about 180° C. to about 210° C. Inaddition to increasing the amount of H₂ present in the anode output, thewater-gas shift reaction can additionally or alternately increase theamount of CO₂ at the expense of CO. This can exchangedifficult-to-remove carbon monoxide (CO) for carbon dioxide, which canbe more readily removed by condensation (e.g., cryogenic removal),chemical reaction (such as amine removal), and/or other CO₂ removalmethods. Additionally or alternately, it may be desirable to increasethe CO content present in the anode exhaust in order to achieve adesired ratio of H₂ to CO.

After passing through the optional water gas shift reaction stage, theanode output can be passed through one or more separation stages forremoval of water and/or CO₂ from the anode output stream. For example,one or more CO₂ output streams can be formed by performing CO₂separation on the anode output using one or more methods individually orin combination. Such methods can be used to generate CO₂ outputstream(s) having a CO₂ content of 90 vol % or greater, such as at least95% vol % CO₂, or at least 98 vol % CO₂. Such methods can recover aboutat least about 70% of the CO₂ content of the anode output, such as atleast about 80% of the CO₂ content of the anode output, or at leastabout 90%. Alternatively, in some aspects it may be desirable to recoveronly a portion of the CO₂ within an anode output stream, with therecovered portion of CO₂ being about 33% to about 90% of the CO₂ in theanode output, such as at least about 40%, or at least about 50%. Forexample, it may be desirable to retain some CO₂ in the anode output flowso that a desired composition can be achieved in a subsequent water gasshift stage. Suitable separation methods may comprise use of a physicalsolvent (e.g., Selexol™ or Rectisol™); amines or other bases (e.g., MEAor MDEA); refrigeration (e.g., cryogenic separation); pressure swingadsorption; vacuum swing adsorption; and combinations thereof. Acryogenic CO₂ separator can be an example of a suitable separator. Asthe anode output is cooled, the majority of the water in the anodeoutput can be separated out as a condensed (liquid) phase. Furthercooling and/or pressurizing of the water-depleted anode output flow canthen separate high purity CO₂, as the other remaining components in theanode output flow (such as H₂, N₂, CH₄) do not tend to readily formcondensed phases. A cryogenic CO₂ separator can recover between about33% and about 90% of the CO₂ present in a flow, depending on theoperating conditions.

Removal of water from the anode exhaust to form one or more water outputstreams can also be beneficial, whether prior to, during, or afterperforming CO₂ separation. The amount of water in the anode output canvary depending on operating conditions selected. For example, thesteam-to-carbon ratio established at the anode inlet can affect thewater content in the anode exhaust, with high steam-to-carbon ratiostypically resulting in a large amount of water that can pass through theanode unreacted and/or reacted only due to the water gas shiftequilibrium in the anode. Depending on the aspect, the water content inthe anode exhaust can correspond to up to about 30% or more of thevolume in the anode exhaust. Additionally or alternately, the watercontent can be about 80% or less of the volume of the anode exhaust.While such water can be removed by compression and/or cooling withresulting condensation, the removal of this water can require extracompressor power and/or heat exchange surface area and excessive coolingwater. One beneficial way to remove a portion of this excess water canbe based on use of an adsorbent bed that can capture the humidity fromthe moist anode effluent and can then be ‘regenerated’ using dry anodefeed gas, in order to provide additional water for the anode feed.HVAC-style (heating, ventilation, and air conditioning) adsorptionwheels design can be applicable, because anode exhaust and inlet can besimilar in pressure, and minor leakage from one stream to the other canhave minimal impact on the overall process. In embodiments where CO₂removal is performed using a cryogenic process, removal of water priorto or during CO₂ removal may be desirable, including removal bytriethyleneglycol (TEG) system and/or desiccants. By contrast, if anamine wash is used for CO₂ removal, water can be removed from the anodeexhaust downstream from the CO₂ removal stage.

Alternately or in addition to a CO₂ output stream and/or a water outputstream, the anode output can be used to form one or more product streamscontaining a desired chemical or fuel product. Such a product stream orstreams can correspond to a syngas stream, a hydrogen stream, or bothsyngas product and hydrogen product streams. For example, a hydrogenproduct stream containing at least about 70 vol % H₂, such as at leastabout 90 vol % H₂ or at least about 95 vol % H₂, can be formed.Additionally or alternately, a syngas stream containing at least about70 vol % of H₂ and CO combined, such as at least about 90 vol % of H₂and CO can be formed. The one or more product streams can have a gasvolume corresponding to at least about 75% of the combined H₂ and CO gasvolumes in the anode output, such as at least about 85% or at leastabout 90% of the combined H₂ and CO gas volumes. It is noted that therelative amounts of H₂ and CO in the products streams may differ fromthe H₂ to CO ratio in the anode output based on use of water gas shiftreaction stages to convert between the products.

In some aspects, it can be desirable to remove or separate a portion ofthe H₂ present in the anode output. For example, in some aspects the H₂to CO ratio in the anode exhaust can be at least about 3.0:1. Bycontrast, processes that make use of syngas, such as Fischer-Tropschsynthesis, may consume H₂ and CO in a different ratio, such as a ratiothat is closer to 2:1. One alternative can be to use a water gas shiftreaction to modify the content of the anode output to have an H₂ to COratio closer to a desired syngas composition. Another alternative can beto use a membrane separation to remove a portion of the H₂ present inthe anode output to achieve a desired ratio of H₂ and CO, or stillalternately to use a combination of membrane separation and water gasshift reactions. One advantage of using a membrane separation to removeonly a portion of the H₂ in the anode output can be that the desiredseparation can be performed under relatively mild conditions. Since onegoal can be to produce a retentate that still has a substantial H₂content, a permeate of high purity hydrogen can be generated by membraneseparation without requiring severe conditions. For example, rather thanhaving a pressure on the permeate side of the membrane of about 100 kPaaor less (such as ambient pressure), the permeate side can be at anelevated pressure relative to ambient while still having sufficientdriving force to perform the membrane separation. Additionally oralternately, a sweep gas such as methane can be used to provide adriving force for the membrane separation. This can reduce the purity ofthe H₂ permeate stream, but may be advantageous, depending on thedesired use for the permeate stream.

In various aspects of the invention, at least a portion of the anodeexhaust stream (preferably after separation of CO₂ and/or H₂O) can beused as a feed for a process external to the fuel cell and associatedreforming stages. In various aspects, the anode exhaust can have a ratioof H₂ to CO of about 1.5:1 to about 10:1, such as at least about 3.0:1,or at least about 4.0:1, or at least about 5.0:1. A syngas stream can begenerated or withdrawn from the anode exhaust. The anode exhaust gas,optionally after separation of CO₂ and/or H₂O, and optionally afterperforming a water gas shift reaction and/or a membrane separation toremove excess hydrogen, can correspond to a stream containingsubstantial portions of H₂ and/or CO. For a stream with a relatively lowcontent of CO, such as a stream where the ratio of H₂ to CO is at leastabout 3:1, the anode exhaust can be suitable for use as an H₂ feed.Examples of processes that could benefit from an H₂ feed can include,but are not limited to, refinery processes, an ammonia synthesis plant,or a turbine in a (different) power generation system, or combinationsthereof. Depending on the application, still lower CO₂ contents can bedesirable. For a stream with an H₂-to-CO ratio of less than about 2.2 to1 and greater than about 1.9 to 1, the stream can be suitable for use asa syngas feed. Examples of processes that could benefit from a syngasfeed can include, but are not limited to, a gas-to-liquids plant (suchas a plant using a Fischer-Tropsch process with a non-shifting catalyst)and/or a methanol synthesis plant. The amount of the anode exhaust usedas a feed for an external process can be any convenient amount.Optionally, when a portion of the anode exhaust is used as a feed for anexternal process, a second portion of the anode exhaust can be recycledto the anode input and/or recycled to the combustion zone for acombustion-powered generator.

The input streams useful for different types of Fischer-Tropschsynthesis processes can provide an example of the different types ofproduct streams that may be desirable to generate from the anode output.For a Fischer-Tropsch synthesis reaction system that uses a shiftingcatalyst, such as an iron-based catalyst, the desired input stream tothe reaction system can include CO₂ in addition to H₂ and CO. If asufficient amount of CO₂ is not present in the input stream, aFischer-Tropsch catalyst with water gas shift activity can consume CO inorder to generate additional CO₂, resulting in a syngas that can bedeficient in CO. For integration of such a Fischer-Tropsch process withan MCFC fuel cell, the separation stages for the anode output can beoperated to retain a desired amount of CO₂ (and optionally H₂O) in thesyngas product. By contrast, for a Fischer-Tropsch catalyst based on anon-shifting catalyst, any CO₂ present in a product stream could serveas an inert component in the Fischer-Tropsch reaction system.

In an aspect where the membrane is swept with a sweep gas such as amethane sweep gas, the methane sweep gas can correspond to a methanestream used as the anode fuel or in a different low pressure process,such as a boiler, furnace, gas turbine, or other fuel-consuming device.In such an aspect, low levels of CO₂ permeation across the membrane canhave minimal consequence. Such CO₂ that may permeate across the membranecan have a minimal impact on the reactions within the anode, and suchCO₂ can remain contained in the anode product. Therefore, the CO₂ (ifany) lost across the membrane due to permeation does not need to betransferred again across the MCFC electrolyte. This can significantlyreduce the separation selectivity requirement for the hydrogenpermeation membrane. This can allow, for example, use of ahigher-permeability membrane having a lower selectivity, which canenable use of a lower pressure and/or reduced membrane surface area. Insuch an aspect of the invention, the volume of the sweep gas can be alarge multiple of the volume of hydrogen in the anode exhaust, which canallow the effective hydrogen concentration on the permeate side to bemaintained close to zero. The hydrogen thus separated can beincorporated into the turbine-fed methane where it can enhance theturbine combustion characteristics, as described above.

It is noted that excess H₂ produced in the anode can represent a fuelwhere the greenhouse gases have already been separated. Any CO₂ in theanode output can be readily separated from the anode output, such as byusing an amine wash, a cryogenic CO₂ separator, and/or a pressure orvacuum swing absorption process. Several of the components of the anodeoutput (H₂, CO, CH₄) are not easily removed, while CO₂ and H₂O canusually be readily removed. Depending on the embodiment, at least about90 vol % of the CO₂ in the anode output can be separated out to form arelatively high purity CO₂ output stream. Thus, any CO₂ generated in theanode can be efficiently separated out to form a high purity CO₂ outputstream. After separation, the remaining portion of the anode output cancorrespond primarily to components with chemical and/or fuel value, aswell as reduced amounts of CO₂ and/or H₂O. Since a substantial portionof the CO₂ generated by the original fuel (prior to reforming) can havebeen separated out, the amount of CO₂ generated by subsequent burning ofthe remaining portion of the anode output can be reduced. In particular,to the degree that the fuel in the remaining portion of the anode outputis H₂, no additional greenhouse gases can typically be formed by burningof this fuel.

The anode exhaust can be subjected to a variety of gas processingoptions, including water-gas shift and separation of the components fromeach other. Two general anode processing schemes are shown in FIGS. 1and 2.

FIG. 1 schematically shows an example of a reaction system for operatinga fuel cell array of molten carbonate fuel cells in conjunction with achemical synthesis process. In FIG. 1, a fuel stream 105 is provided toa reforming stage (or stages) 110 associated with the anode 127 of afuel cell 120, such as a fuel cell that is part of a fuel cell stack ina fuel cell array. The reforming stage 110 associated with fuel cell 120can be internal to a fuel cell assembly. In some optional aspects, anexternal reforming stage (not shown) can also be used to reform aportion of the reformable fuel in an input stream prior to passing theinput stream into a fuel cell assembly. Fuel stream 105 can preferablyinclude a reformable fuel, such as methane, other hydrocarbons, and/orother hydrocarbon-like compounds such as organic compounds containingcarbon-hydrogen bonds. Fuel stream 105 can also optionally contain H₂and/or CO, such as H₂ and/or CO provided by optional anode recyclestream 185. It is noted that anode recycle stream 185 is optional, andthat in many aspects no recycle stream is provided from the anodeexhaust 125 back to anode 127, either directly or indirectly viacombination with fuel stream 105 or reformed fuel stream 115. Afterreforming, the reformed fuel stream 115 can be passed into anode 127 offuel cell 120. A CO₂ and O₂-containing stream 119 can also be passedinto cathode 129. A flow of carbonate ions 122, CO₃ ²⁻, from the cathodeportion 129 of the fuel cell can provide the remaining reactant neededfor the anode fuel cell reactions. Based on the reactions in the anode127, the resulting anode exhaust 125 can include H₂O, CO₂, one or morecomponents corresponding to incompletely reacted fuel (H₂, CO, CH₄, orother components corresponding to a reformable fuel), and optionally oneor more additional nonreactive components, such as N₂ and/or othercontaminants that are part of fuel stream 105. The anode exhaust 125 canthen be passed into one or more separation stages. For example, a CO₂removal stage 140 can correspond to a cryogenic CO₂ removal system, anamine wash stage for removal of acid gases such as CO₂, or anothersuitable type of CO₂ separation stage for separating a CO₂ output stream143 from the anode exhaust. Optionally, the anode exhaust can first bepassed through a water gas shift reactor 130 to convert any CO presentin the anode exhaust (along with some H₂O) into CO₂ and H₂ in anoptionally water gas shifted anode exhaust 135. Depending on the natureof the CO₂ removal stage, a water condensation or removal stage 150 maybe desirable to remove a water output stream 153 from the anode exhaust.Though shown in FIG. 1 after the CO₂ separation stage 140, it mayoptionally be located before the CO₂ separation stage 140 instead.Additionally, an optional membrane separation stage 160 for separationof H₂ can be used to generate a high purity permeate stream 163 of H₂.The resulting retentate stream 166 can then be used as an input to achemical synthesis process. Stream 166 could additionally or alternatelybe shifted in a second water-gas shift reactor 131 to adjust the H₂, CO,and CO₂ content to a different ratio, producing an output stream 168 forfurther use in a chemical synthesis process. In FIG. 1, anode recyclestream 185 is shown as being withdrawn from the retentate stream 166,but the anode recycle stream 185 could additionally or alternately bewithdrawn from other convenient locations in or between the variousseparation stages. The separation stages and shift reactor(s) couldadditionally or alternately be configured in different orders, and/or ina parallel configuration. Finally, a stream with a reduced content ofCO₂ 139 can be generated as an output from cathode 129. For the sake ofsimplicity, various stages of compression and heat addition/removal thatmight be useful in the process, as well as steam addition or removal,are not shown.

As noted above, the various types of separations performed on the anodeexhaust can be performed in any convenient order. FIG. 2 shows anexample of an alternative order for performing separations on an anodeexhaust. In FIG. 2, anode exhaust 125 can be initially passed intoseparation stage 260 for removing a portion 263 of the hydrogen contentfrom the anode exhaust 125. This can allow, for example, reduction ofthe H₂ content of the anode exhaust to provide a retentate 266 with aratio of H₂ to CO closer to 2:1. The ratio of H₂ to CO can then befurther adjusted to achieve a desired value in a water gas shift stage230. The water gas shifted output 235 can then pass through CO₂separation stage 240 and water removal stage 250 to produce an outputstream 275 suitable for use as an input to a desired chemical synthesisprocess. Optionally, output stream 275 could be exposed to an additionalwater gas shift stage (not shown). A portion of output stream 275 canoptionally be recycled (not shown) to the anode input. Of course, stillother combinations and sequencing of separation stages can be used togenerate a stream based on the anode output that has a desiredcomposition. For the sake of simplicity, various stages of compressionand heat addition/removal that might be useful in the process, as wellas steam addition or removal, are not shown.

Cathode Inputs and Outputs

Conventionally, a molten carbonate fuel cell can be operated based ondrawing a desired load while consuming some portion of the fuel in thefuel stream delivered to the anode. The voltage of the fuel cell canthen be determined by the load, fuel input to the anode, air and CO₂provided to the cathode, and the internal resistances of the fuel cell.The CO₂ to the cathode can be conventionally provided in part by usingthe anode exhaust as at least a part of the cathode input stream. Bycontrast, the present invention can use separate/different sources forthe anode input and cathode input. By removing any direct link betweenthe composition of the anode input flow and the cathode input flow,additional options become available for operating the fuel cell, such asto generate excess synthesis gas, to improve capture of carbon dioxide,and/or to improve the total efficiency (electrical plus chemical power)of the fuel cell, among others.

In a molten carbonate fuel cell, the transport of carbonate ions acrossthe electrolyte in the fuel cell can provide a method for transportingCO₂ from a first flow path to a second flow path, where the transportmethod can allow transport from a lower concentration (the cathode) to ahigher concentration (the anode), which can thus facilitate capture ofCO₂. Part of the selectivity of the fuel cell for CO₂ separation can bebased on the electrochemical reactions allowing the cell to generateelectrical power. For nonreactive species (such as N₂) that effectivelydo not participate in the electrochemical reactions within the fuelcell, there can be an insignificant amount of reaction and transportfrom cathode to anode. By contrast, the potential (voltage) differencebetween the cathode and anode can provide a strong driving force fortransport of carbonate ions across the fuel cell. As a result, thetransport of carbonate ions in the molten carbonate fuel cell can allowCO₂ to be transported from the cathode (lower CO₂ concentration) to theanode (higher CO₂ concentration) with relatively high selectivity.However, a challenge in using molten carbonate fuel cells for carbondioxide removal can be that the fuel cells have limited ability toremove carbon dioxide from relatively dilute cathode feeds. The voltageand/or power generated by a carbonate fuel cell can start to droprapidly as the CO₂ concentration falls below about 2.0 vol %. As the CO₂concentration drops further, e.g., to below about 1.0 vol %, at somepoint the voltage across the fuel cell can become low enough that littleor no further transport of carbonate may occur and the fuel cell ceasesto function. Thus, at least some CO₂ is likely to be present in theexhaust gas from the cathode stage of a fuel cell under commerciallyviable operating conditions.

The amount of carbon dioxide delivered to the fuel cell cathode(s) canbe determined based on the CO₂ content of a source for the cathodeinlet. One example of a suitable CO₂-containing stream for use as acathode input flow can be an output or exhaust flow from a combustionsource. Examples of combustion sources include, but are not limited to,sources based on combustion of natural gas, combustion of coal, and/orcombustion of other hydrocarbon-type fuels (including biologicallyderived fuels). Additional or alternate sources can include other typesof boilers, fired heaters, furnaces, and/or other types of devices thatburn carbon-containing fuels in order to heat another substance (such aswater or air). To a first approximation, the CO₂ content of the outputflow from a combustion source can be a minor portion of the flow. Evenfor a higher CO₂ content exhaust flow, such as the output from acoal-fired combustion source, the CO₂ content from most commercialcoal-fired power plants can be about 15 vol % or less. More generally,the CO₂ content of an output or exhaust flow from a combustion sourcecan be at least about 1.5 vol %, or at least about 1.6 vol %, or atleast about 1.7 vol %, or at least about 1.8 vol %, or at least about1.9 vol %, or at least greater 2 vol %, or at least about 4 vol %, or atleast about 5 vol %, or at least about 6 vol %, or at least about 8 vol%. Additionally or alternately, the CO₂ content of an output or exhaustflow from a combustion source can be about 20 vol % or less, such asabout 15 vol % or less, or about 12 vol % or less, or about 10 vol % orless, or about 9 vol % or less, or about 8 vol % or less, or about 7 vol% or less, or about 6.5 vol % or less, or about 6 vol % or less, orabout 5.5 vol % or less, or about 5 vol % or less, or about 4.5 vol % orless. The concentrations given above are on a dry basis. It is notedthat the lower CO₂ content values can be present in the exhaust fromsome natural gas or methane combustion sources, such as generators thatare part of a power generation system that may or may not include anexhaust gas recycle loop.

Other potential sources for a cathode input stream can additionally oralternately include sources of bio-produced CO₂. This can include, forexample, CO₂ generated during processing of bio-derived compounds, suchas CO₂ generated during ethanol production. An additional or alternateexample can include CO₂ generated by combustion of a bio-produced fuel,such as combustion of lignocellulose. Still other additional oralternate potential CO₂ sources can correspond to output or exhauststreams from various industrial processes, such as CO₂-containingstreams generated by plants for manufacture of steel, cement, and/orpaper.

Yet another additional or alternate potential source of CO₂ can beCO₂-containing streams from a fuel cell. The CO₂-containing stream froma fuel cell can correspond to a cathode output stream from a differentfuel cell, an anode output stream from a different fuel cell, a recyclestream from the cathode output to the cathode input of a fuel cell,and/or a recycle stream from an anode output to a cathode input of afuel cell. For example, an MCFC operated in standalone mode underconventional conditions can generate a cathode exhaust with a CO₂concentration of at least about 5 vol %. Such a CO₂-containing cathodeexhaust could be used as a cathode input for an MCFC operated accordingto an aspect of the invention. More generally, other types of fuel cellsthat generate a CO₂ output from the cathode exhaust can additionally oralternately be used, as well as other types of CO₂-containing streamsnot generated by a “combustion” reaction and/or by a combustion-poweredgenerator. Optionally but preferably, a CO₂-containing stream fromanother fuel cell can be from another molten carbonate fuel cell. Forexample, for molten carbonate fuel cells connected in series withrespect to the cathodes, the output from the cathode for a first moltencarbonate fuel cell can be used as the input to the cathode for a secondmolten carbonate fuel cell.

For various types of CO₂-containing streams from sources other thancombustion sources, the CO₂ content of the stream can vary widely. TheCO₂ content of an input stream to a cathode can contain at least about 2vol % of CO₂, such as at least about 4 vol %, or at least about 5 vol %,or at least about 6 vol %, or at least about 8 vol %. Additionally oralternately, the CO₂ content of an input stream to a cathode can beabout 30 vol % or less, such as about 25 vol % or less, or about 20 vol% or less, or about 15 vol % or less, or about 10 vol % or less, orabout 8 vol % or less, or about 6 vol % or less, or about 4 vol % orless. For some still higher CO₂ content streams, the CO₂ content can begreater than about 30 vol %, such as a stream substantially composed ofCO₂ with only incidental amounts of other compounds. As an example, agas-fired turbine without exhaust gas recycle can produce an exhauststream with a CO₂ content of approximately 4.2 vol %. With EGR, agas-fired turbine can produce an exhaust stream with a CO₂ content ofabout 6-8 vol %. Stoichiometric combustion of methane can produce anexhaust stream with a CO₂ content of about 11 vol %. Combustion of coalcan produce an exhaust stream with a CO₂ content of about 15-20 vol %.Fired heaters using refinery off-gas can produce an exhaust stream witha CO₂ content of about 12-15 vol %. A gas turbine operated on a low BTUgas without any EGR can produce an exhaust stream with a CO₂ content of˜12 vol %.

In addition to CO₂, a cathode input stream must include O₂ to providethe components necessary for the cathode reaction. Some cathode inputstreams can be based on having air as a component. For example, acombustion exhaust stream can be formed by combusting a hydrocarbon fuelin the presence of air. Such a combustion exhaust stream, or anothertype of cathode input stream having an oxygen content based on inclusionof air, can have an oxygen content of about 20 vol % or less, such asabout 15 vol % or less, or about 10 vol % or less. Additionally oralternately, the oxygen content of the cathode input stream can be atleast about 4 vol %, such as at least about 6 vol %, or at least about 8vol %. More generally, a cathode input stream can have a suitablecontent of oxygen for performing the cathode reaction. In some aspects,this can correspond to an oxygen content of about 5 vol % to about 15vol %, such as from about 7 vol % to about 9 vol %. For many types ofcathode input streams, the combined amount of CO₂ and O₂ can correspondto less than about 21 vol % of the input stream, such as less than about15 vol % of the stream or less than about 10 vol % of the stream. An airstream containing oxygen can be combined with a CO₂ source that has lowoxygen content. For example, the exhaust stream generated by burningcoal may include a low oxygen content that can be mixed with air to forma cathode inlet stream.

In addition to CO₂ and O₂, a cathode input stream can also be composedof inert/non-reactive species such as N₂, H₂O, and other typical oxidant(air) components. For example, for a cathode input derived from anexhaust from a combustion reaction, if air is used as part of theoxidant source for the combustion reaction, the exhaust gas can includetypical components of air such as N₂, H₂O, and other compounds in minoramounts that are present in air. Depending on the nature of the fuelsource for the combustion reaction, additional species present aftercombustion based on the fuel source may include one or more of H₂O,oxides of nitrogen (NOx) and/or sulfur (SOx), and other compounds eitherpresent in the fuel and/or that are partial or complete combustionproducts of compounds present in the fuel, such as CO. These species maybe present in amounts that do not poison the cathode catalyst surfacesthough they may reduce the overall cathode activity. Such reductions inperformance may be acceptable, or species that interact with the cathodecatalyst may be reduced to acceptable levels by known pollutant removaltechnologies.

The amount of O₂ present in a cathode input stream (such as an inputcathode stream based on a combustion exhaust) can advantageously besufficient to provide the oxygen needed for the cathode reaction in thefuel cell. Thus, the volume percentage of O₂ can advantageously be atleast 0.5 times the amount of CO₂ in the exhaust. Optionally, asnecessary, additional air can be added to the cathode input to providesufficient oxidant for the cathode reaction. When some form of air isused as the oxidant, the amount of N₂ in the cathode exhaust can be atleast about 78 vol %, e.g., at least about 88 vol %, and/or about 95 vol% or less. In some aspects, the cathode input stream can additionally oralternately contain compounds that are generally viewed as contaminants,such as H₂S or NH₃. In other aspects, the cathode input stream can becleaned to reduce or minimize the content of such contaminants.

In addition to the reaction to form carbonate ions for transport acrossthe electrolyte, the conditions in the cathode can also be suitable forconversion of nitrogen oxides into nitrate and/or nitrate ions.Hereinafter, only nitrate ions will be referred to for convenience. Theresulting nitrate ions can also be transported across the electrolytefor reaction in the anode. NOx concentrations in a cathode input streamcan typically be on the order of ppm, so this nitrate transport reactioncan have a minimal impact on the amount of carbonate transported acrossthe electrolyte. However, this method of NOx removal can be beneficialfor cathode input streams based on combustion exhausts from gasturbines, as this can provide a mechanism for reducing NOx emissions.The conditions in the cathode can additionally or alternately besuitable for conversion of unburned hydrocarbons (in combination with O₂in the cathode input stream) to typical combustion products, such as CO₂and H₂O.

A suitable temperature for operation of an MCFC can be between about450° C. and about 750° C., such as at least about 500° C., e.g., with aninlet temperature of about 550° C. and an outlet temperature of about625° C. Prior to entering the cathode, heat can be added to or removedfrom the combustion exhaust, if desired, e.g., to provide heat for otherprocesses, such as reforming the fuel input for the anode. For example,if the source for the cathode input stream is a combustion exhauststream, the combustion exhaust stream may have a temperature greaterthan a desired temperature for the cathode inlet. In such an aspect,heat can be removed from the combustion exhaust prior to use as thecathode input stream. Alternatively, the combustion exhaust could be atvery low temperature, for example after a wet gas scrubber on acoal-fired boiler, in which case the combustion exhaust can be belowabout 100° C. Alternatively, the combustion exhaust could be from theexhaust of a gas turbine operated in combined cycle mode, in which thegas can be cooled by raising steam to run a steam turbine for additionalpower generation. In this case, the gas can be below about 50° C. Heatcan be added to a combustion exhaust that is cooler than desired.

Fuel Cell Arrangement

In various aspects, a configuration option for a fuel cell (such as afuel cell array containing multiple fuel cell stacks) can be to dividethe CO₂-containing stream between a plurality of fuel cells. Some typesof sources for CO₂-containing streams can generate large volumetric flowrates relative to the capacity of an individual fuel cell. For example,the CO₂-containing output stream from an industrial combustion sourcecan typically correspond to a large flow volume relative to desirableoperating conditions for a single MCFC of reasonable size. Instead ofprocessing the entire flow in a single MCFC, the flow can be dividedamongst a plurality of MCFC units, usually at least some of which can bein parallel, so that the flow rate in each unit can be within a desiredflow range.

A second configuration option can be to utilize fuel cells in series tosuccessively remove CO₂ from a flow stream. Regardless of the number ofinitial fuel cells to which a CO₂-containing stream can be distributedto in parallel, each initial fuel cell can be followed by one or moreadditional cells in series to further remove additional CO₂. If thedesired amount of CO₂ in the cathode output is sufficiently low,attempting to remove CO₂ from a cathode input stream down to the desiredlevel in a single fuel cell or fuel cell stage could lead to a lowand/or unpredictable voltage output for the fuel cell. Rather thanattempting to remove CO₂ to the desired level in a single fuel cell orfuel cell stage, CO₂ can be removed in successive cells until a desiredlevel can be achieved. For example, each cell in a series of fuel cellscan be used to remove some percentage (e.g., about 50%) of the CO₂present in a fuel stream. In such an example, if three fuel cells areused in series, the CO₂ concentration can be reduced (e.g., to about 15%or less of the original amount present, which can correspond to reducingthe CO₂ concentration from about 6% to about 1% or less over the courseof three fuel cells in series).

In another configuration, the operating conditions can be selected inearly fuel stages in series to provide a desired output voltage whilethe array of stages can be selected to achieve a desired level of carbonseparation. As an example, an array of fuel cells can be used with threefuel cells in series. The first two fuel cells in series can be used toremove CO₂ while maintaining a desired output voltage. The final fuelcell can then be operated to remove CO₂ to a desired concentration butat a lower voltage.

In still another configuration, there can be separate connectivity forthe anodes and cathodes in a fuel cell array. For example, if the fuelcell array includes fuel cathodes connected in series, the correspondinganodes can be connected in any convenient manner, not necessarilymatching up with the same arrangement as their corresponding cathodes,for example. This can include, for instance, connecting the anodes inparallel, so that each anode receives the same type of fuel feed, and/orconnecting the anodes in a reverse series, so that the highest fuelconcentration in the anodes can correspond to those cathodes having thelowest CO₂ concentration.

In yet another configuration, the amount of fuel delivered to one ormore anode stages and/or the amount of CO₂ delivered to one or morecathode stages can be controlled in order to improve the performance ofthe fuel cell array. For example, a fuel cell array can have a pluralityof cathode stages connected in series. In an array that includes threecathode stages in series, this can mean that the output from a firstcathode stage can correspond to the input for a second cathode stage,and the output from the second cathode stage can correspond to the inputfor a third cathode stage. In this type of configuration, the CO₂concentration can decrease with each successive cathode stage. Tocompensate for this reduced CO₂ concentration, additional hydrogenand/or methane can be delivered to the anode stages corresponding to thelater cathode stages. The additional hydrogen and/or methane in theanodes corresponding to the later cathode stages can at least partiallyoffset the loss of voltage and/or current caused by the reduced CO₂concentration, which can increase the voltage and thus net powerproduced by the fuel cell. In another example, the cathodes in a fuelcell array can be connected partially in series and partially inparallel. In this type of example, instead of passing the entirecombustion output into the cathodes in the first cathode stage, at leasta portion of the combustion exhaust can be passed into a later cathodestage. This can provide an increased CO₂ content in a later cathodestage. Still other options for using variable feeds to either anodestages or cathode stages can be used if desired.

The cathode of a fuel cell can correspond to a plurality of cathodesfrom an array of fuel cells, as previously described. In some aspects, afuel cell array can be operated to improve or maximize the amount ofcarbon transferred from the cathode to the anode. In such aspects, forthe cathode output from the final cathode(s) in an array sequence(typically at least including a series arrangement, or else the finalcathode(s) and the initial cathode(s) would be the same), the outputcomposition can include about 2.0 vol % or less of CO₂ (e.g., about 1.5vol % or less or about 1.2 vol % or less) and/or at least about 1.0 vol% of CO₂, such as at least about 1.2 vol % or at least about 1.5 vol %.Due to this limitation, the net efficiency of CO₂ removal when usingmolten carbonate fuel cells can be dependent on the amount of CO₂ in thecathode input. For cathode input streams with CO₂ contents of greaterthan about 6 vol %, such as at least about 8%, the limitation on theamount of CO₂ that can be removed is not severe. However, for acombustion reaction using natural gas as a fuel and with excess air, asis typically found in a gas turbine, the amount of CO₂ in the combustionexhaust may only correspond to a CO₂ concentration at the cathode inputof less than about 5 vol %. Use of exhaust gas recycle can allow theamount of CO₂ at the cathode input to be increased to at least about 5vol %, e.g., at least about 6 vol %. If EGR is increased when usingnatural gas as a fuel to produce a CO₂ concentration beyond about 6 vol%, then the flammability in the combustor can be decreased and the gasturbine may become unstable. However, when H₂ is added to the fuel, theflammability window can be significantly increased, allowing the amountof exhaust gas recycle to be increased further, so that concentrationsof CO₂ at the cathode input of at least about 7.5 vol % or at leastabout 8 vol % can be achieved. As an example, based on a removal limitof about 1.5 vol % at the cathode exhaust, increasing the CO₂ content atthe cathode input from about 5.5 vol % to about 7.5 vol % can correspondto a ˜10% increase in the amount of CO₂ that can be captured using afuel cell and transported to the anode loop for eventual CO₂ separation.The amount of O₂ in the cathode output can additionally or alternatelybe reduced, typically in an amount proportional to the amount of CO₂removed, which can result in small corresponding increases in theamount(s) of the other (non-cathode-reactive) species at the cathodeexit.

In other aspects, a fuel cell array can be operated to improve ormaximize the energy output of the fuel cell, such as the total energyoutput, the electric energy output, the syngas chemical energy output,or a combination thereof. For example, molten carbonate fuel cells canbe operated with an excess of reformable fuel in a variety ofsituations, such as for generation of a syngas stream for use inchemical synthesis plant and/or for generation of a high purity hydrogenstream. The syngas stream and/or hydrogen stream can be used as a syngassource, a hydrogen source, as a clean fuel source, and/or for any otherconvenient application. In such aspects, the amount of CO₂ in thecathode exhaust can be related to the amount of CO₂ in the cathode inputstream and the CO₂ utilization at the desired operating conditions forimproving or maximizing the fuel cell energy output.

Additionally or alternately, depending on the operating conditions, anMCFC can lower the CO₂ content of a cathode exhaust stream to about 5.0vol % or less, e.g., about 4.0 vol % or less, or about 2.0 vol % orless, or about 1.5 vol % or less, or about 1.2 vol % or less.Additionally or alternately, the CO₂ content of the cathode exhauststream can be at least about 0.9 vol %, such as at least about 1.0 vol%, or at least about 1.2 vol %, or at least about 1.5 vol %.

Molten Carbonate Fuel Cell Operation

In some aspects, a fuel cell may be operated in a single pass oronce-through mode. In single pass mode, reformed products in the anodeexhaust are not returned to the anode inlet. Thus, recycling syngas,hydrogen, or some other product from the anode output directly to theanode inlet is not done in single pass operation. More generally, insingle pass operation, reformed products in the anode exhaust are alsonot returned indirectly to the anode inlet, such as by using reformedproducts to process a fuel stream subsequently introduced into the anodeinlet. Optionally, CO₂ from the anode outlet can be recycled to thecathode inlet during operation of an MCFC in single pass mode. Moregenerally, in some alternative aspects, recycling from the anode outletto the cathode inlet may occur for an MCFC operating in single passmode. Heat from the anode exhaust or output may additionally oralternately be recycled in a single pass mode. For example, the anodeoutput flow may pass through a heat exchanger that cools the anodeoutput and warms another stream, such as an input stream for the anodeand/or the cathode. Recycling heat from anode to the fuel cell isconsistent with use in single pass or once-through operation. Optionallybut not preferably, constituents of the anode output may be burned toprovide heat to the fuel cell during single pass mode.

FIG. 3 shows a schematic example of the operation of an MCFC forgeneration of electrical power. In FIG. 3, the anode portion of the fuelcell can receive fuel and steam (H₂O) as inputs, with outputs of water,CO₂, and optionally excess H₂, CH₄ (or other hydrocarbons), and/or CO.The cathode portion of the fuel cell can receive CO₂ and some oxidant(e.g., air/O₂) as inputs, with an output corresponding to a reducedamount of CO₂ in O₂-depleted oxidant (air). Within the fuel cell, CO₃ ²⁻ions formed in the cathode side can be transported across theelectrolyte to provide the carbonate ions needed for the reactionsoccurring at the anode.

Several reactions can occur within a molten carbonate fuel cell such asthe example fuel cell shown in FIG. 3. The reforming reactions can beoptional, and can be reduced or eliminated if sufficient H₂ is provideddirectly to the anode. The following reactions are based on CH₄, butsimilar reactions can occur when other fuels are used in the fuel cell.

<anode reforming>CH₄+H₂O=>3H₂+CO  (1)

<water gas shift>CO+H₂O=>H₂+CO₂  (2)

<reforming and water gas shift combined>CH₄+2H₂O=>4H₂+CO₂  (3)

<anode H₂ oxidation>H₂+CO₃ ²⁻=>H₂O+CO₂+2e ⁻  (4)

<cathode>½O₂+CO₂+2e ⁻=>CO₃ ²⁻  (5)

Reaction (1) represents the basic hydrocarbon reforming reaction togenerate H₂ for use in the anode of the fuel cell. The CO formed inreaction (1) can be converted to H₂ by the water-gas shift reaction (2).The combination of reactions (1) and (2) is shown as reaction (3).Reactions (1) and (2) can occur external to the fuel cell, and/or thereforming can be performed internal to the anode.

Reactions (4) and (5), at the anode and cathode respectively, representthe reactions that can result in electrical power generation within thefuel cell. Reaction (4) combines H₂, either present in the feed oroptionally generated by reactions (1) and/or (2), with carbonate ions toform H₂O, CO₂, and electrons to the circuit. Reaction (5) combines O₂,CO₂, and electrons from the circuit to form carbonate ions. Thecarbonate ions generated by reaction (5) can be transported across theelectrolyte of the fuel cell to provide the carbonate ions needed forreaction (4). In combination with the transport of carbonate ions acrossthe electrolyte, a closed current loop can then be formed by providingan electrical connection between the anode and cathode.

In various embodiments, a goal of operating the fuel cell can be toimprove the total efficiency of the fuel cell and/or the totalefficiency of the fuel cell plus an integrated chemical synthesisprocess. This is typically in contrast to conventional operation of afuel cell, where the goal can be to operate the fuel cell with highelectrical efficiency for using the fuel provided to the cell forgeneration of electrical power. As defined above, total fuel cellefficiency may be determined by dividing the electric output of the fuelcell plus the lower heating value of the fuel cell outputs by the lowerheating value of the input components for the fuel cell. In other words,TFCE=(LHV(el)+LHV(sg out))/LHV(in), where LHV(in) and LHV(sg out) referto the LHV of the fuel components (such as H₂, CH₄, and/or CO) deliveredto the fuel cell and syngas (H₂, CO and/or CO₂) in the anode outletstreams or flows, respectively. This can provide a measure of theelectric energy plus chemical energy generated by the fuel cell and/orthe integrated chemical process. It is noted that under this definitionof total efficiency, heat energy used within the fuel cell and/or usedwithin the integrated fuel cell/chemical synthesis system can contributeto total efficiency. However, any excess heat exchanged or otherwisewithdrawn from the fuel cell or integrated fuel cell/chemical synthesissystem is excluded from the definition. Thus, if excess heat from thefuel cell is used, for example, to generate steam for electricitygeneration by a steam turbine, such excess heat is excluded from thedefinition of total efficiency.

Several operational parameters may be manipulated to operate a fuel cellwith excess reformable fuel. Some parameters can be similar to thosecurrently recommended for fuel cell operation. In some aspects, thecathode conditions and temperature inputs to the fuel cell can besimilar to those recommended in the literature. For example, the desiredelectrical efficiency and the desired total fuel cell efficiency may beachieved at a range of fuel cell operating temperatures typical formolten carbonate fuel cells. In typical operation, the temperature canincrease across the fuel cell.

In other aspects, the operational parameters of the fuel cell candeviate from typical conditions so that the fuel cell is operated toallow a temperature decrease from the anode inlet to the anode outletand/or from the cathode inlet to the cathode outlet. For example, thereforming reaction to convert a hydrocarbon into H₂ and CO is anendothermic reaction. If a sufficient amount of reforming is performedin a fuel cell anode relative to the amount of oxidation of hydrogen togenerate electrical current, the net heat balance in the fuel cell canbe endothermic. This can cause a temperature drop between the inlets andoutlets of a fuel cell. During endothermic operation, the temperaturedrop in the fuel cell can be controlled so that the electrolyte in thefuel cell remains in a molten state.

Parameters that can be manipulated in a way so as to differ from thosecurrently recommended can include the amount of fuel provided to theanode, the composition of the fuel provided to the anode, and/or theseparation and capture of syngas in the anode output without significantrecycling of syngas from the anode exhaust to either the anode input orthe cathode input. In some aspects, no recycle of syngas or hydrogenfrom the anode exhaust to either the anode input or the cathode inputcan be allowed to occur, either directly or indirectly. In additional oralternative aspects, a limited amount of recycle can occur. In suchaspects, the amount of recycle from the anode exhaust to the anode inputand/or the cathode input can be less than about 10 vol % of the anodeexhaust, such as less than about 5 vol %, or less than about 1 vol %.

Additionally or alternately, a goal of operating a fuel cell can be toseparate CO₂ from the output stream of a combustion reaction or anotherprocess that produces a CO₂ output stream, in addition to allowinggeneration of electric power. In such aspects, the combustionreaction(s) can be used to power one or more generators or turbines,which can provide a majority of the power generated by the combinedgenerator/fuel cell system. Rather than operating the fuel cell tooptimize power generation by the fuel cell, the system can instead beoperated to improve the capture of carbon dioxide from thecombustion-powered generator while reducing or minimizing the number offuels cells required for capturing the carbon dioxide. Selecting anappropriate configuration for the input and output flows of the fuelcell, as well as selecting appropriate operating conditions for the fuelcell, can allow for a desirable combination of total efficiency andcarbon capture.

In some embodiments, the fuel cells in a fuel cell array can be arrangedso that only a single stage of fuel cells (such as fuel cell stacks) canbe present. In this type of embodiment, the anode fuel utilization forthe single stage can represent the anode fuel utilization for the array.Another option can be that a fuel cell array can contain multiple stagesof anodes and multiple stages of cathodes, with each anode stage havinga fuel utilization within the same range, such as each anode stagehaving a fuel utilization within 10% of a specified value, for examplewithin 5% of a specified value. Still another option can be that eachanode stage can have a fuel utilization equal to a specified value orlower than the specified value by less than an amount, such as havingeach anode stage be not greater than a specified value by 10% or less,for example, by 5% or less. As an illustrative example, a fuel cellarray with a plurality of anode stages can have each anode stage bewithin about 10% of 50% fuel utilization, which would correspond to eachanode stage having a fuel utilization between about 40% and about 60%.As another example, a fuel cell array with a plurality of stages canhave each anode stage be not greater than 60% anode fuel utilizationwith the maximum deviation being about 5% less, which would correspondto each anode stage having a fuel utilization between about 55% to about60%. In still another example, one or more stages of fuel cells in afuel cell array can be operated at a fuel utilization from about 30% toabout 50%, such as operating a plurality of fuel cell stages in thearray at a fuel utilization from about 30% to about 50%. More generally,any of the above types of ranges can be paired with any of the anodefuel utilization values specified herein.

Still another additional or alternate option can include specifying afuel utilization for less than all of the anode stages. For example, insome aspects of the invention fuel cells/stacks can be arranged at leastpartially in one or more series arrangements such that anode fuelutilization can be specified for the first anode stage in a series, thesecond anode stage in a series, the final anode stage in a series, orany other convenient anode stage in a series. As used herein, the“first” stage in a series corresponds to the stage (or set of stages, ifthe arrangement contains parallel stages as well) to which input isdirectly fed from the fuel source(s), with later (“second,” “third,”“final,” etc.) stages representing the stages to which the output fromone or more previous stages is fed, instead of directly from therespective fuel source(s). In situations where both output from previousstages and input directly from the fuel source(s) are co-fed into astage, there can be a “first” (set of) stage(s) and a “last” (set of)stage(s), but other stages (“second,” “third,” etc.) can be more trickyamong which to establish an order (e.g., in such cases, ordinal ordercan be determined by concentration levels of one or more components inthe composite input feed composition, such as CO₂ for instance, fromhighest concentration “first” to lowest concentration “last” withapproximately similar compositional distinctions representing the sameordinal level.)

Yet another additional or alternate option can be to specify the anodefuel utilization corresponding to a particular cathode stage (again,where fuel cells/stacks can be arranged at least partially in one ormore series arrangements). As noted above, based on the direction of theflows within the anodes and cathodes, the first cathode stage may notcorrespond to (be across the same fuel cell membrane from) the firstanode stage. Thus, in some aspects of the invention, the anode fuelutilization can be specified for the first cathode stage in a series,the second cathode stage in a series, the final cathode stage in aseries, or any other convenient cathode stage in a series.

Yet still another additional or alternate option can be to specify anoverall average of fuel utilization over all fuel cells in a fuel cellarray. In various aspects, the overall average of fuel utilization for afuel cell array can be about 65% or less, for example, about 60% orless, about 55% or less, about 50% or less, or about 45% or less(additionally or alternately, the overall average fuel utilization for afuel cell array can be at least about 25%, for example at least about30%, at least about 35%, or at least about 40%). Such an average fuelutilization need not necessarily constrain the fuel utilization in anysingle stage, so long as the array of fuel cells meets the desired fuelutilization.

Applications for CO₂ Output after Capture

In various aspects of the invention, the systems and methods describedabove can allow for production of carbon dioxide as a pressurized fluid.For example, the CO₂ generated from a cryogenic separation stage caninitially correspond to a pressurized CO₂ liquid with a purity of atleast about 90%, e.g., at least about 95%, at least about 97%, at leastabout 98%, or at least about 99%. This pressurized CO₂ stream can beused, e.g., for injection into wells in order to further enhance oil orgas recovery such as in secondary oil recovery. When done in proximityto a facility that encompasses a gas turbine, the overall system maybenefit from additional synergies in use of electrical/mechanical powerand/or through heat integration with the overall system.

Alternatively, for systems dedicated to an enhanced oil recovery (EOR)application (i.e., not comingled in a pipeline system with tightcompositional standards), the CO₂ separation requirements may besubstantially relaxed. The EOR application can be sensitive to thepresence of O₂, so O₂ can be absent, in some embodiments, from a CO₂stream intended for use in EOR. However, the EOR application can tend tohave a low sensitivity to dissolved CO, H₂, and/or CH₄. Also, pipelinesthat transport the CO₂ can be sensitive to these impurities. Thosedissolved gases can typically have only subtle impacts on thesolubilizing ability of CO₂ used for EOR. Injecting gases such as CO,H₂, and/or CH₄ as EOR gases can result in some loss of fuel valuerecovery, but such gases can be otherwise compatible with EORapplications.

Additionally or alternately, a potential use for CO₂ as a pressurizedliquid can be as a nutrient in biological processes such as algaegrowth/harvesting. The use of MCFCs for CO₂ separation can ensure thatmost biologically significant pollutants could be reduced to acceptablylow levels, resulting in a CO₂-containing stream having only minoramounts of other “contaminant” gases (such as CO, H₂, N₂, and the like,and combinations thereof) that are unlikely to substantially negativelyaffect the growth of photosynthetic organisms. This can be in starkcontrast to the output streams generated by most industrial sources,which can often contain potentially highly toxic material such as heavymetals.

In this type of aspect of the invention, the CO₂ stream generated byseparation of CO₂ in the anode loop can be used to produce biofuelsand/or chemicals, as well as precursors thereof. Further additionally oralternately, CO₂ may be produced as a dense fluid, allowing for mucheasier pumping and transport across distances, e.g., to large fields ofphotosynthetic organisms. Conventional emission sources can emit hot gascontaining modest amounts of CO₂ (e.g., about 4-15%) mixed with othergases and pollutants. These materials would normally need to be pumpedas a dilute gas to an algae pond or biofuel “farm”. By contrast, theMCFC system according to the invention can produce a concentrated CO₂stream (˜60-70% by volume on a dry basis) that can be concentratedfurther to 95%+(for example 96%+, 97%+, 98%+, or 99%+) and easilyliquefied. This stream can then be transported easily and efficientlyover long distances at relatively low cost and effectively distributedover a wide area. In these embodiments, residual heat from thecombustion source/MCFC may be integrated into the overall system aswell.

An alternative embodiment may apply where the CO₂ source/MCFC andbiological/chemical production sites are co-located. In that case, onlyminimal compression may be necessary (i.e., to provide enough CO₂pressure to use in the biological production, e.g., from about 15 psigto about 150 psig). Several novel arrangements can be possible in such acase. Secondary reforming may optionally be applied to the anode exhaustto reduce CH₄ content, and water-gas shift may optionally additionallyor alternately be present to drive any remaining CO into CO₂ and H₂.

The components from an anode output stream and/or cathode output streamcan be used for a variety of purposes. One option can be to use theanode output as a source of hydrogen, as described above. For an MCFCintegrated with or co-located with a refinery, the hydrogen can be usedas a hydrogen source for various refinery processes, such ashydroprocessing. Another option can be to additionally or alternatelyuse hydrogen as a fuel source where the CO₂ from combustion has alreadybeen “captured.” Such hydrogen can be used in a refinery or otherindustrial setting as a fuel for a boiler, furnace, and/or fired heater,and/or the hydrogen can be used as a feed for an electric powergenerator, such as a turbine. Hydrogen from an MCFC fuel cell canfurther additionally or alternately be used as an input stream for othertypes of fuel cells that require hydrogen as an input, possiblyincluding vehicles powered by fuel cells. Still another option can be toadditionally or alternately use syngas generated as an output from anMCFC fuel cell as a fermentation input.

Another option can be to additionally or alternately use syngasgenerated from the anode output. Of course, syngas can be used as afuel, although a syngas based fuel can still lead to some CO₂ productionwhen burned as fuel. In other aspects, a syngas output stream can beused as an input for a chemical synthesis process. One option can be toadditionally or alternately use syngas for a Fischer-Tropsch typeprocess, and/or another process where larger hydrocarbon molecules areformed from the syngas input. Another option can be to additionally oralternately use syngas to form an intermediate product such as methanol.Methanol could be used as the final product, but in other aspectsmethanol generated from syngas can be used to generate larger compounds,such as gasoline, olefins, aromatics, and/or other products. It is notedthat a small amount of CO₂ can be acceptable in the syngas feed to amethanol synthesis process, and/or to a Fischer-Tropsch processutilizing a shifting catalyst. Hydroformylation is an additional oralternate example of still another synthesis process that can make useof a syngas input.

It is noted that one variation on use of an MCFC to generate syngas canbe to use MCFC fuel cells as part of a system for processing methaneand/or natural gas withdrawn by an offshore oil platform or otherproduction system that is a considerable distance from its ultimatemarket. Instead of attempting to transport the gas phase output from awell, or attempting to store the gas phase product for an extendedperiod, the gas phase output from a well can be used as the input to anMCFC fuel cell array. This can lead to a variety of benefits. First, theelectric power generated by the fuel cell array can be used as a powersource for the platform. Additionally, the syngas output from the fuelcell array can be used as an input for a Fischer-Tropsch process at theproduction site. This can allow for formation of liquid hydrocarbonproducts more easily transported by pipeline, ship, or railcar from theproduction site to, for example, an on-shore facility or a largerterminal.

Still other integration options can additionally or alternately includeusing the cathode output as a source of higher purity, heated nitrogen.The cathode input can often include a large portion of air, which meansa substantial portion of nitrogen can be included in the cathode input.The fuel cell can transport CO₂ and O₂ from the cathode across theelectrolyte to the anode, and the cathode outlet can have lowerconcentrations of CO₂ and O₂, and thus a higher concentration of N₂ thanfound in air. With subsequent removal of the residual O₂ and CO₂, thisnitrogen output can be used as an input for production of ammonia orother nitrogen-containing chemicals, such as urea, ammonium nitrate,and/or nitric acid. It is noted that urea synthesis could additionallyor alternately use CO₂ separate from the anode output as an input feed.

Integration Example Applications for Integration with CombustionTurbines

In some aspects of the invention, a combustion source for generatingpower and exhausting a CO₂-containing exhaust can be integrated with theoperation of molten carbonate fuel cells. An example of a suitablecombustion source is a gas turbine. Preferably, the gas turbine cancombust natural gas, methane gas, or another hydrocarbon gas in acombined cycle mode integrated with steam generation and heat recoveryfor additional efficiency. Modern natural gas combined cycleefficiencies are about 60% for the largest and newest designs. Theresulting CO₂-containing exhaust gas stream can be produced at anelevated temperature compatible with the MCFC operation, such as 300°C.-700° C. and preferably 500° C.-650° C. The gas source can optionallybut preferably be cleaned of contaminants such as sulfur that can poisonthe MCFC before entering the turbine. Alternatively, the gas source canbe a coal-fired generator, wherein the exhaust gas would typically becleaned post-combustion due to the greater level of contaminants in theexhaust gas. In such an alternative, some heat exchange to/from the gasmay be necessary to enable clean-up at lower temperatures. In additionalor alternate embodiments, the source of the CO₂-containing exhaust gascan be the output from a boiler, combustor, or other heat source thatburns carbon-rich fuels. In other additional or alternate embodiments,the source of the CO₂-containing exhaust gas can be bio-produced CO₂ incombination with other sources.

For integration with a combustion source, some alternativeconfigurations for processing of a fuel cell anode can be desirable. Forexample, an alternative configuration can be to recycle at least aportion of the exhaust from a fuel cell anode to the input of a fuelcell anode. The output stream from an MCFC anode can include H₂O, CO₂,optionally CO, and optionally but typically unreacted fuel (such as H₂or CH₄) as the primary output components. Instead of using this outputstream as an external fuel stream and/or an input stream for integrationwith another process, one or more separations can be performed on theanode output stream in order to separate the CO₂ from the componentswith potential fuel value, such as H₂ or CO. The components with fuelvalue can then be recycled to the input of an anode.

This type of configuration can provide one or more benefits. First, CO₂can be separated from the anode output, such as by using a cryogenic CO₂separator. Several of the components of the anode output (H₂, CO, CH₄)are not easily condensable components, while CO₂ and H₂O can beseparated individually as condensed phases. Depending on the embodiment,at least about 90 vol % of the CO₂ in the anode output can be separatedto form a relatively high purity CO₂ output stream. Alternatively, insome aspects less CO₂ can be removed from the anode output, so thatabout 50 vol % to about 90 vol % of the CO₂ in the anode output can beseparated out, such as about 80 vol % or less or about 70 vol % or less.After separation, the remaining portion of the anode output cancorrespond primarily to components with fuel value, as well as reducedamounts of CO₂ and/or H₂O. This portion of the anode output afterseparation can be recycled for use as part of the anode input, alongwith additional fuel. In this type of configuration, even though thefuel utilization in a single pass through the MCFC(s) may be low, theunused fuel can be advantageously recycled for another pass through theanode. As a result, the single-pass fuel utilization can be at a reducedlevel, while avoiding loss (exhaust) of unburned fuel to theenvironment.

Additionally or alternatively to recycling a portion of the anodeexhaust to the anode input, another configuration option can be to use aportion of the anode exhaust as an input for a combustion reaction for aturbine or other combustion device, such as a boiler, furnace, and/orfired heater. The relative amounts of anode exhaust recycled to theanode input and/or as an input to the combustion device can be anyconvenient or desirable amount. If the anode exhaust is recycled to onlyone of the anode input and the combustion device, the amount of recyclecan be any convenient amount, such as up to 100% of the portion of theanode exhaust remaining after any separation to remove CO₂ and/or H₂O.When a portion of the anode exhaust is recycled to both the anode inputand the combustion device, the total recycled amount by definition canbe 100% or less of the remaining portion of anode exhaust. Otherwise,any convenient split of the anode exhaust can be used. In variousembodiments of the invention, the amount of recycle to the anode inputcan be at least about 10% of the anode exhaust remaining afterseparations, for example at least about 25%, at least about 40%, atleast about 50%, at least about 60%, at least about 75%, or at leastabout 90%. Additionally or alternately in those embodiments, the amountof recycle to the anode input can be about 90% or less of the anodeexhaust remaining after separations, for example about 75% or less,about 60% or less, about 50% or less, about 40% or less, about 25% orless, or about 10% or less. Further additionally or alternately, invarious embodiments of the invention, the amount of recycle to thecombustion device can be at least about 10% of the anode exhaustremaining after separations, for example at least about 25%, at leastabout 40%, at least about 50%, at least about 60%, at least about 75%,or at least about 90%. Additionally or alternately in those embodiments,the amount of recycle to the combustion device can be about 90% or lessof the anode exhaust remaining after separations, for example about 75%or less, about 60% or less, about 50% or less, about 40% or less, about25% or less, or about 10% or less.

In still other alternative aspects of the invention, the fuel for acombustion device can additionally or alternately be a fuel with anelevated quantity of components that are inert and/or otherwise act as adiluent in the fuel. CO₂ and N₂ are examples of components in a naturalgas feed that can be relatively inert during a combustion reaction. Whenthe amount of inert components in a fuel feed reaches a sufficientlevel, the performance of a turbine or other combustion source can beimpacted. The impact can be due in part to the ability of the inertcomponents to absorb heat, which can tend to quench the combustionreaction. Examples of fuel feeds with a sufficient level of inertcomponents can include fuel feeds containing at least about 20 vol %CO₂, or fuel feeds containing at least about 40 vol % N₂, or fuel feedscontaining combinations of CO₂ and N₂ that have sufficient inert heatcapacity to provide similar quenching ability. (It is noted that CO₂ hasa greater heat capacity than N₂, and therefore lower concentrations ofCO₂ can have a similar impact as higher concentrations of N₂. CO₂ canalso participate in the combustion reactions more readily than N₂, andin doing so remove H₂ from the combustion. This consumption of H₂ canhave a large impact on the combustion of the fuel, by reducing the flamespeed and narrowing the flammability range of the air and fuel mixture.)More generally, for a fuel feed containing inert components that impactthe flammability of the fuel feed, the inert components in the fuel feedcan be at least about 20 vol %, such as at least about 40 vol %, or atleast about 50 vol %, or at least about 60 vol %. Preferably, the amountof inert components in the fuel feed can be about 80 vol % or less.

When a sufficient amount of inert components are present in a fuel feed,the resulting fuel feed can be outside of the flammability window forthe fuel components of the feed. In this type of situation, addition ofH₂ from a recycled portion of the anode exhaust to the combustion zonefor the generator can expand the flammability window for the combinationof fuel feed and H₂, which can allow, for example, a fuel feedcontaining at least about 20 vol % CO₂ or at least about 40% N₂ (orother combinations of CO₂ and N₂) to be successfully combusted.

Relative to a total volume of fuel feed and H₂ delivered to a combustionzone, the amount of H₂ for expanding the flammability window can be atleast about 5 vol % of the total volume of fuel feed plus H₂, such as atleast about 10 vol %, and/or about 25 vol % or less. Another option forcharacterizing the amount of H₂ to add to expand the flammability windowcan be based on the amount of fuel components present in the fuel feedbefore H₂ addition. Fuel components can correspond to methane, naturalgas, other hydrocarbons, and/or other components conventionally viewedas fuel for a combustion-powered turbine or other generator. The amountof H₂ added to the fuel feed can correspond to at least about one thirdof the volume of fuel components (1:3 ratio of H₂:fuel component) in thefuel feed, such as at least about half of the volume of the fuelcomponents (1:2 ratio). Additionally or alternately, the amount of H₂added to the fuel feed can be roughly equal to the volume of fuelcomponents in the fuel feed (1:1 ratio) or less. For example, for a feedcontaining about 30 vol % CH₄, about 10% N₂, and about 60% CO₂, asufficient amount of anode exhaust can be added to the fuel feed toachieve about a 1:2 ratio of H₂ to CH₄. For an idealized anode exhaustthat contained only H₂, addition of H₂ to achieve a 1:2 ratio wouldresult in a feed containing about 26 vol % CH₄, 13 vol % H₂, 9 vol % N₂,and 52 vol % CO₂.

Exhaust Gas Recycle

Aside from providing exhaust gas to a fuel cell array for capture andeventual separation of the CO₂, an additional or alternate potential usefor exhaust gas can include recycle back to the combustion reaction toincrease the CO₂ content. When hydrogen is available for addition to thecombustion reaction, such as hydrogen from the anode exhaust of the fuelcell array, further benefits can be gained from using recycled exhaustgas to increase the CO₂ content within the combustion reaction.

In various aspects of the invention, the exhaust gas recycle loop of apower generation system can receive a first portion of the exhaust gasfrom combustion, while the fuel cell array can receive a second portion.The amount of exhaust gas from combustion recycled to the combustionzone of the power generation system can be any convenient amount, suchas at least about 15% (by volume), for example at least about 25%, atleast about 35%, at least about 45%, or at least about 50%. Additionallyor alternately, the amount of combustion exhaust gas recirculated to thecombustion zone can be about 65% (by volume) or less, e.g., about 60% orless, about 55% or less, about 50% or less, or about 45% or less.

In one or more aspects of the invention, a mixture of an oxidant (suchas air and/or oxygen-enriched air) and fuel can be combusted and(simultaneously) mixed with a stream of recycled exhaust gas. The streamof recycled exhaust gas, which can generally include products ofcombustion such as CO₂, can be used as a diluent to control, adjust, orotherwise moderate the temperature of combustion and of the exhaust thatcan enter the succeeding expander. As a result of using oxygen-enrichedair, the recycled exhaust gas can have an increased CO₂ content, therebyallowing the expander to operate at even higher expansion ratios for thesame inlet and discharge temperatures, thereby enabling significantlyincreased power production.

A gas turbine system can represent one example of a power generationsystem where recycled exhaust gas can be used to enhance the performanceof the system. The gas turbine system can have a first/main compressorcoupled to an expander via a shaft. The shaft can be any mechanical,electrical, or other power coupling, thereby allowing a portion of themechanical energy generated by the expander to drive the maincompressor. The gas turbine system can also include a combustion chamberconfigured to combust a mixture of a fuel and an oxidant. In variousaspects of the invention, the fuel can include any suitable hydrocarbongas/liquid, such as syngas, natural gas, methane, ethane, propane,butane, naphtha diesel, kerosene, aviation fuel, coal derived fuel,bio-fuel, oxygenated hydrocarbon feedstock, or any combinations thereof.The oxidant can, in some embodiments, be derived from a second or inletcompressor fluidly coupled to the combustion chamber and adapted tocompress a feed oxidant. In one or more embodiments of the invention,the feed oxidant can include atmospheric air and/or enriched air. Whenthe oxidant includes enriched air alone or a mixture of atmospheric airand enriched air, the enriched air can be compressed by the inletcompressor (in the mixture, either before or after being mixed with theatmospheric air). The enriched air and/or the air-enriched air mixturecan have an overall oxygen concentration of at least about 25 volume %,e.g., at least about 30 volume %, at least about 35 volume %, at leastabout 40 volume %, at least about 45 volume %, or at least about 50volume %. Additionally or alternately, the enriched air and/or theair-enriched air mixture can have an overall oxygen concentration ofabout 80 volume % or less, such as about 70 volume % or less.

The enriched air can be derived from any one or more of several sources.For example, the enriched air can be derived from such separationtechnologies as membrane separation, pressure swing adsorption,temperature swing adsorption, nitrogen plant-byproduct streams, and/orcombinations thereof. The enriched air can additionally or alternatelybe derived from an air separation unit (ASU), such as a cryogenic ASU,for producing nitrogen for pressure maintenance or other purposes. Incertain embodiments of the invention, the reject stream from such an ASUcan be rich in oxygen, having an overall oxygen content from about 50volume % to about 70 volume %, can be used as at least a portion of theenriched air and subsequently diluted, if needed, with unprocessedatmospheric air to obtain the desired oxygen concentration.

In addition to the fuel and oxidant, the combustion chamber canoptionally also receive a compressed recycle exhaust gas, such as anexhaust gas recirculation primarily having CO₂ and nitrogen components.The compressed recycle exhaust gas can be derived from the maincompressor, for instance, and adapted to help facilitate combustion ofthe oxidant and fuel, e.g., by moderating the temperature of thecombustion products. As can be appreciated, recirculating the exhaustgas can serve to increase CO₂ concentration.

An exhaust gas directed to the inlet of the expander can be generated asa product of combustion reaction. The exhaust gas can have a heightenedCO₂ content based, at least in part, on the introduction of recycledexhaust gas into the combustion reaction. As the exhaust gas expandsthrough the expander, it can generate mechanical power to drive the maincompressor, to drive an electrical generator, and/or to power otherfacilities.

The power generation system can, in many embodiments, also include anexhaust gas recirculation (EGR) system. In one or more aspects of theinvention, the EGR system can include a heat recovery steam generator(HRSG) and/or another similar device fluidly coupled to a steam gasturbine. In at least one embodiment, the combination of the HRSG and thesteam gas turbine can be characterized as a power-producing closedRankine cycle. In combination with the gas turbine system, the HRSG andthe steam gas turbine can form part of a combined-cycle power generatingplant, such as a natural gas combined-cycle (NGCC) plant. The gaseousexhaust can be introduced to the HRSG in order to generate steam and acooled exhaust gas. The HRSG can include various units for separatingand/or condensing water out of the exhaust stream, transferring heat toform steam, and/or modifying the pressure of streams to a desired level.In certain embodiments, the steam can be sent to the steam gas turbineto generate additional electrical power.

After passing through the HRSG and optional removal of at least someH₂O, the CO₂-containing exhaust stream can, in some embodiments, berecycled for use as an input to the combustion reaction. As noted above,the exhaust stream can be compressed (or decompressed) to match thedesired reaction pressure within the vessel for the combustion reaction.

Example of Integrated System

FIG. 4 schematically shows an example of an integrated system includingintroduction of both CO₂-containing recycled exhaust gas and H₂ or COfrom the fuel cell anode exhaust into the combustion reaction forpowering a turbine. In FIG. 4, the turbine can include a compressor 402,a shaft 404, an expander 406, and a combustion zone 415. An oxygensource 411 (such as air and/or oxygen-enriched air) can be combined withrecycled exhaust gas 498 and compressed in compressor 402 prior toentering combustion zone 415. A fuel 412, such as CH₄, and optionally astream containing H₂ or CO 187 can be delivered to the combustion zone.The fuel and oxidant can be reacted in zone 415 and optionally butpreferably passed through expander 406 to generate electric power. Theexhaust gas from expander 106 can be used to form two streams, e.g., aCO₂-containing stream 422 (that can be used as an input feed for fuelcell array 425) and another CO₂-containing stream 492 (that can be usedas the input for a heat recovery and steam generator system 490, whichcan, for example, enable additional electricity to be generated usingsteam turbines 494). After passing through heat recovery system 490,including optional removal of a portion of H₂O from the CO₂-containingstream, the output stream 498 can be recycled for compression incompressor 402 or a second compressor that is not shown. The proportionof the exhaust from expander 406 used for CO₂-containing stream 492 canbe determined based on the desired amount of CO₂ for addition tocombustion zone 415.

As used herein, the EGR ratio is the flow rate for the fuel cell boundportion of the exhaust gas divided by the combined flow rate for thefuel cell bound portion and the recovery bound portion, which is sent tothe heat recovery generator. For example, the EGR ratio for flows shownin FIG. 4 is the flow rate of stream 422 divided by the combined flowrate of streams 422 and 492.

The CO₂-containing stream 422 can be passed into a cathode portion (notshown) of a molten carbonate fuel cell array 425. Based on the reactionswithin fuel cell array 425, CO₂ can be separated from stream 422 andtransported to the anode portion (not shown) of the fuel cell array 425.This can result in a cathode output stream 424 depleted in CO₂. Thecathode output stream 424 can then be passed into a heat recovery (andoptional steam generator) system 450 for generation of heat exchangeand/or additional generation of electricity using steam turbines 454(which may optionally be the same as the aforementioned steam turbines494). After passing through heat recovery and steam generator system450, the resulting flue gas stream 456 can be exhausted to theenvironment and/or passed through another type of carbon capturetechnology, such as an amine scrubber.

After transport of CO₂ from the cathode side to the anode side of fuelcell array 425, the anode output 435 can optionally be passed into awater gas shift reactor 470. Water gas shift reactor 470 can be used togenerate additional H₂ and CO₂ at the expense of CO (and H₂O) present inthe anode output 435. The output from the optional water gas shiftreactor 470 can then be passed into one or more separation stages 440,such as a cold box or a cryogenic separator. This can allow forseparation of an H₂O stream 447 and CO₂ stream 449 from the remainingportion of the anode output. The remaining portion of the anode output485 can include unreacted H₂ generated by reforming but not consumed infuel cell array 425. A first portion 445 of the H₂-containing stream 485can be recycled to the input for the anode(s) in fuel cell array 425. Asecond portion 487 of stream 485 can be used as an input for combustionzone 415. A third portion 465 can be used as is for another purposeand/or treated for subsequent further use. Although FIG. 4 and thedescription herein schematically details up to three portions, it iscontemplated that only one of these three portions can be exploited,only two can be exploited, or all three can be exploited according tothe invention.

In FIG. 4, the exhaust for the exhaust gas recycle loop is provided by afirst heat recovery and steam generator system 490, while a second heatrecovery and steam generator system 450 can be used to capture excessheat from the cathode output of the fuel cell array 425. FIG. 5 shows analternative embodiment where the exhaust gas recycle loop is provided bythe same heat recovery steam generator used for processing the fuel cellarray output. In FIG. 5, recycled exhaust gas 598 is provided by heatrecovery and steam generator system 550 as a portion of the flue gasstream 556. This can eliminate the separate heat recovery and steamgenerator system associated with the turbine.

In various embodiments of the invention, the process can be approachedas starting with a combustion reaction for powering a turbine, aninternal combustion engine, or another system where heat and/or pressuregenerated by a combustion reaction can be converted into another form ofpower. The fuel for the combustion reaction can comprise or be hydrogen,a hydrocarbon, and/or any other compound containing carbon that can beoxidized (combusted) to release energy. Except for when the fuelcontains only hydrogen, the composition of the exhaust gas from thecombustion reaction can have a range of CO₂ contents, depending on thenature of the reaction (e.g., from at least about 2 vol % to about 25vol % or less). Thus, in certain embodiments where the fuel iscarbonaceous, the CO₂ content of the exhaust gas can be at least about 2vol %, for example at least about 4 vol %, at least about 5 vol %, atleast about 6 vol %, at least about 8 vol %, or at least about 10 vol %.Additionally or alternately in such carbonaceous fuel embodiments, theCO₂ content can be about 25 vol % or less, for example about 20 vol % orless, about 15 vol % or less, about 10 vol % or less, about 7 vol % orless, or about 5 vol % or less. Exhaust gases with lower relative CO₂contents (for carbonaceous fuels) can correspond to exhaust gases fromcombustion reactions on fuels such as natural gas with lean (excess air)combustion. Higher relative CO₂ content exhaust gases (for carbonaceousfuels) can correspond to optimized natural gas combustion reactions,such as those with exhaust gas recycle, and/or combustion of fuels suchas coal.

In some aspects of the invention, the fuel for the combustion reactioncan contain at least about 90 volume % of compounds containing fivecarbons or less, e.g., at least about 95 volume %. In such aspects, theCO₂ content of the exhaust gas can be at least about 4 vol %, forexample at least about 5 vol %, at least about 6 vol %, at least about 7vol %, or at least about 7.5 vol %. Additionally or alternately, the CO₂content of the exhaust gas can be about 13 vol % or less, e.g., about 12vol % or less, about 10 vol % or less, about 9 vol % or less, about 8vol % or less, about 7 vol % or less, or about 6 vol % or less. The CO₂content of the exhaust gas can represent a range of values depending onthe configuration of the combustion-powered generator. Recycle of anexhaust gas can be beneficial for achieving a CO₂ content of at leastabout 6 vol %, while addition of hydrogen to the combustion reaction canallow for further increases in CO₂ content to achieve a CO₂ content ofat least about 7.5 vol %.

Alternative Configuration—High Severity NOx Turbine

Gas turbines can be limited in their operation by several factors. Onetypical limitation can be that the maximum temperature in the combustionzone can be controlled below certain limits to achieve sufficiently lowconcentrations of nitrogen oxides (NOx) in order to satisfy regulatoryemission limits. Regulatory emission limits can require a combustionexhaust to have a NOx content of about 20 vppm or less, and possible 10vppm or less, when the combustion exhaust is allowed to exit to theenvironment.

NOx formation in natural gas-fired combustion turbines can be a functionof temperature and residence time. Reactions that result in formation ofNOx can be of reduced and/or minimal importance below a flametemperature of about 1500° F., but NOx production can increase rapidlyas the temperature increases beyond this point. In a gas turbine,initial combustion products can be mixed with extra air to cool themixture to a temperature around 1200° F., and temperature can be limitedby the metallurgy of the expander blades. Early gas turbines typicallyexecuted the combustion in diffusion flames that had stoichiometriczones with temperatures well above 1500° F., resulting in higher NOxconcentrations. More recently, the current generation of ‘Dry Low Nox’(DLN) burners can use special pre-mixed burners to burn natural gas atcooler lean (less fuel than stoichiometric) conditions. For example,more of the dilution air can be mixed in to the initial flame, and lesscan be mixed in later to bring the temperature down to the ˜1200° F.turbine-expander inlet temperature. The disadvantages for DLN burnerscan include poor performance at turndown, higher maintenance, narrowranges of operation, and poor fuel flexibility. The latter can be aconcern, as DLN burners can be more difficult to apply to fuels ofvarying quality (or difficult to apply at all to liquid fuels). For lowBTU fuels, such as fuels containing a high content of CO₂, DLN burnersare typically not used and instead diffusion burners can be used. Inaddition, gas turbine efficiency can be increased by using a higherturbine-expander inlet temperature. However, because there can be alimited amount of dilution air, and this amount can decrease withincreased turbine-expander inlet temperature, the DLN burner can becomeless effective at maintaining low NOx as the efficiency of the gasturbine improves.

In various aspects of the invention, a system integrating a gas turbinewith a fuel cell for carbon capture can allow use of higher combustionzone temperatures while reducing and/or minimizing additional NOxemissions, as well as enabling DLN-like NOx savings via use of turbinefuels that are not presently compatible with DLN burners. In suchaspects, the turbine can be run at higher power (i.e., highertemperature) resulting in higher NOx emissions, but also higher poweroutput and potentially higher efficiency. In some aspects of theinvention, the amount of NOx in the combustion exhaust can be at leastabout 20 vppm, such as at least about 30 vppm, or at least about 40vppm. Additionally or alternately, the amount of NOx in the combustionexhaust can be about 1000 vppm or less, such as about 500 vppm or less,or about 250 vppm or less, or about 150 vppm or less, or about 100 vppmor less. In order to reduce the NOx levels to levels required byregulation, the resulting NOx can be equilibrated via thermal NOxdestruction (reduction of NOx levels to equilibrium levels in theexhaust stream) through one of several mechanisms, such as simplethermal destruction in the gas phase; catalyzed destruction from thenickel cathode catalyst in the fuel cell array; and/or assisted thermaldestruction prior to the fuel cell by injection of small amounts ofammonia, urea, or other reductant. This can be assisted by introductionof hydrogen derived from the anode exhaust. Further reduction of NOx inthe cathode of the fuel cell can be achieved via electrochemicaldestruction wherein the NOx can react at the cathode surface and can bedestroyed. This can result in some nitrogen transport across themembrane electrolyte to the anode, where it may form ammonia or otherreduced nitrogen compounds. With respect to NOx reduction methodsinvolving an MCFC, the expected NOx reduction from a fuel cell/fuel cellarray can be about 80% or less of the NOx in the input to the fuel cellcathode, such as about 70% or less, and/or at least about 5%. It isnoted that sulfidic corrosion can also limit temperatures and affectturbine blade metallurgy in conventional systems. However, the sulfurrestrictions of the MCFC system can typically require reduced fuelsulfur levels that reduce or minimize concerns related to sulfidiccorrosion. Operating the MCFC array at low fuel utilization can furthermitigate such concerns, such as in aspects where a portion of the fuelfor the combustion reaction corresponds to hydrogen from the anodeexhaust.

Operating the Fuel Cell at Low Voltage

The conventional fuel cell practice teaches that molten carbonate andsolid oxide fuel cells should be operated to maximize power density. Theability to maximize power density can be limited by a need to satisfyother operating constraints, such as temperature differential across thefuel cell. Generally, fuel cell parameters are selected to optimizepower density as much as is feasible given other constraints. As anexample, FIG. 6-13 of the NETL Fuel Cell Handbook and the discussionsurrounding FIG. 6-13 teach that operation of a fuel cell at low fuelutilization is hindered by the decrease in fuel conversion that occursas the fuel utilization is decreased. Generally, a higher operatingvoltage V_(A) is desired to increase power density.

An aspect of the invention can be to operate the fuel cell at low fuelutilization, and to overcome the problem of decreased CH₄ conversion bydecreasing the voltage. The decreased voltage can increase the amount ofheat available for use in the conversion reactions. In various aspects,the fuel cell can be operated at a voltage V_(A) of less than about 0.7Volts, for example less than about 0.68 V, less than about 0.67 V, lessthan about 0.66 V, or about 0.65 V or less. Additionally oralternatively, the fuel cell can be operated at a voltage V_(A) of atleast about 0.60, for example at least about 0.61, at least about 0.62,or at least about 0.63. In so doing, energy that would otherwise leavethe fuel cell as electrical energy at high voltage can remain within thecell as heat as the voltage is lowered. This additional heat can allowfor increased endothermic reactions to occur, for example increasing theCH₄ conversion to syngas.

A series of simulations were performed to illustrate the benefits ofoperating a molten carbonate fuel cell according to the invention.Specifically, the simulations were performed to illustrate the benefitof running the fuel cell at lower voltage across different fuelutilizations. The impact of running the fuel cell at lower voltage andlow fuel utilization is shown in FIGS. 12 and 13. FIG. 12 illustrates amodel of the fuel cell in a representation analogous to FIG. 6-13 of theNETL Fuel Cell Handbook. The simulations used to produce the resultsshown in FIG. 12 were run at a constant CH₄ flow rate. FIG. 12 shows theconversion 1220 that can occur at different fuel utilization 1210percentages for different operating voltages. At high voltage (0.8V)1250, as the fuel utilization is decreased, the CH₄ conversion alsoappeared to be decreased to a low level. As the voltage is lowered (to0.7V, 1240, and 0.6V, 1230), the CH₄ conversion at each fuel utilizationpoint modeled appeared to be higher than the corresponding conversion at0.8V. While FIG. 12 shows only a few percentage increase in CH₄conversion, the impact can actually be quite substantial, as illustratedin FIG. 13.

The simulations used to produce the results shown in FIG. 13 were notperformed at a constant flow rate of CH₄, but at a constant fuel cellarea instead. In FIG. 13, the same operation of the fuel cell wasillustrated not on a percentage of CH₄ conversion basis, but on anabsolute flow rate of CH₄ for a fixed fuel cell area. The x-axis 1310shows the fuel utilization and the y-axis 1320 shows normalized CH₄conversion. Plot 1330 shows simulated results produced at 0.6V. Plot1340 shows the simulated results produced at 0.7V. Plot 1350 shows thesimulated results produced at 0.8V. As the fuel utilization isdecreased, and especially as the voltage is decreased, the currentdensity appeared to be increased by more than a factor of 5 for the datashown in FIGS. 12 and 13. As such, the power density can be increased bylowering the operating voltage under operating conditions consistentwith aspects of the invention. The increased power density and lowervoltage seems to be contrary to the affect achieved during conventionaloperations, where lower operating voltage typically results in lowerpower density. As shown in FIG. 13, the impact on total CH₄ conversionappeared significant: much higher conversion of CH₄, measured as anabsolute flow rate, was achieved at lower fuel utilization when thevoltage was decreased.

ADDITIONAL EMBODIMENTS Embodiment 1

A method for producing electricity, and hydrogen or syngas, using amolten carbonate fuel cell comprising an anode and a cathode, the methodcomprising: introducing an anode fuel stream comprising a reformablefuel into the anode of the molten carbonate fuel cell, an internalreforming element associated with the anode of the molten carbonate fuelcell, or a combination thereof; introducing a cathode inlet streamcomprising CO₂ and O₂ into the cathode of the molten carbonate fuelcell; generating electricity within the molten carbonate fuel cell;generating an anode exhaust from an anode outlet of the molten carbonatefuel cell; separating from the anode exhaust a H₂-containing stream, asyngas-containing stream, or a combination thereof, wherein a fuelutilization of the anode is about 50% or less and a CO₂ utilization ofthe cathode is at least about 60%.

Embodiment 2

The method of Embodiment 1, wherein a reformable hydrogen content of thereformable fuel introduced into the anode of the molten carbonate fuelcell, the internal reforming element associated with the anode of themolten carbonate fuel cell, or the combination thereof, is at leastabout 75% greater than the amount of H₂ oxidized in the anode of themolten carbonate fuel cell to generate electricity.

Embodiment 3

The method of any of the above Embodiments, wherein the cathode inletstream comprises about 20 vol % CO₂ or less (e.g., about 15 vol % CO₂ orless, or about 12 vol % CO₂ or less).

Embodiment 4

The method of any of the above Embodiments, wherein the fuel utilizationof the anode of the molten carbonate fuel cell is about 40% or less(e.g., about 30% or less).

Embodiment 5

The method of any of the above Embodiments, wherein the CO₂ utilizationof the cathode of the molten carbonate fuel cell is at least about 65%(e.g., at least about 70%).

Embodiment 6

The method of any of the above Embodiments, wherein the anode fuelstream comprises at least about 10 vol % inert compounds, at least about10 vol % CO₂, or a combination thereof.

Embodiment 7

The method of any of the above Embodiments, wherein thesyngas-containing stream has a molar ratio of H₂ to CO of about 3.0:1(e.g., about 2.5:1 or less) to about 1.0:1 (e.g., at least about 1.5:1).

Embodiment 8

The method of any of the above Embodiments, wherein the anode exhausthas a molar ratio of H₂ to CO of about 1.5:1 (e.g., at least about3.0:1) to about 10:1.

Embodiment 9

The method of any of the above Embodiments, wherein a) less than 10 vol% of the anode exhaust b) less than 10 vol % of H₂ produced in the anodeof the molten carbonate fuel cell in a single pass or c) less than 10vol % of the syngas-containing stream is directly or indirectly recycledto the anode of the molten carbonate fuel cell or the cathode of themolten carbonate fuel cell.

Embodiment 10

The method of any of Embodiments 1-8, wherein no portion of the anodeexhaust is directly or indirectly recycled to the anode of the moltencarbonate fuel cell, directly or indirectly recycled to the cathode ofthe molten carbonate fuel cell, or a combination thereof.

Embodiment 11

The method of any of the above Embodiments, further comprisingseparating at least one of CO₂ and H₂O from one or a combination of i)the anode exhaust, ii) the H₂-containing stream, and iii) thesyngas-containing stream.

Embodiment 12

The method of any of the above Embodiments, wherein the H₂-containingstream contains at least about 90 vol % H₂ (e.g., at least about 95 vol%, or at least about 98 vol %).

Embodiment 13

The method of any of the above Embodiments, wherein the cathode inletstream comprises a combustion exhaust stream from a combustion-poweredgenerator.

Embodiment 14

The method of any of the above Embodiments, wherein the molten carbonatefuel cell is operated at a voltage V_(A) of about 0.67 Volts or less(e.g, about 0.65 Volts or less).

Example

A series of simulations were performed to illustrate the benefits ofoperating a molten carbonate fuel cell according to the invention. Afirst simulation was performed using a conventional “balanced” fuel cellconfiguration. In a “balanced” configuration, the CO₂ needed foroperation of the cathode was provided at least in part by using theexhaust from the anode, optionally after additional combustion of anyremaining fuel in the anode exhaust. In such a conventionalconfiguration, the amount of fuel delivered to the anode was constrainedbased on the linked nature of the anode exhaust and the cathode input.This balanced configuration was in contrast to the additionalsimulations, where the input flow to the cathode was not directlyrelated to the nature of the anode exhaust. In the additionalsimulations, the fuel utilization in the anode was varied.

In the simulations, the operation of a model molten carbonate fuel cellwas simulated. The amount of reforming and water gas shift occurring inthe fuel cell was modeled as being in equilibrium based on the averagefuel cell temperature. Although a burner was simulated to providetemperature for maintaining the temperature of the fuel cell based onthe extent of the reactions in the fuel cell, heat transport was nototherwise modeled. Thus, the model in the simulation did not attempt tocapture the effect of radiative heat loss by the fuel cell.

In the simulations, a base case corresponding to a conventionalconfiguration was simulated to provide comparative data. FIG. 6 shows anexample of the conventional configuration used in the simulations foroperation of a molten carbonate fuel cell in a “balanced” configuration.In FIG. 6, a source of oxidant (air) 673 and fuel (methane) 675 wereintroduced into a burner 679. The exhaust 625 from the fuel cell anodewas also introduced into the burner. The burner was operated to combustany fuel in the combined streams to generate heat for maintaining thetemperature of fuel cell 620 during operation. Additionally, thecombustion exhaust 629 was passed into the cathode portion of fuel cell620 to provide the CO₂ and O₂ needed for the cathode reaction. In thesimulations, a source of water and methane 605 in an about 2:1 ratio wasintroduced into the anode portion of fuel cell 620. In the simulations,it was assumed that the reforming of the methane provided to the anodewas sufficient to allow the anode reaction to progress without requiringreforming prior to the anode. The fuel cell 620 generated a cathodeexhaust 639 and an anode exhaust 625. The anode exhaust 625 was returnedto the burner.

In addition to the base case, a variety of simulations were performedusing a configuration similar to FIG. 1. The simulation configurationincluded a burner (as shown in FIG. 6) to provide heat for the fuel celland to provide a CO₂ and O₂ containing stream for the cathode. Forconvenience in the simulation, the simulation configuration alsodiffered from FIG. 1 in that a portion of the CO₂ from the anode exhaustcould be returned to the cathode inlet, if the amount of CO₂ exiting theburner was not sufficient. Unlike FIG. 6, however, the configurationsimilar to FIG. 1 did not include any recycle of the full anode exhaustback to either the burner or the cathode inlet.

FIG. 7 shows results from the simulations. In FIG. 7, various initialparameters for the simulation are provided in the Fuel Cell Parameterssection. The first four rows show the values used to determine the CO₂utilization in each simulation. For the simulations in FIG. 7, theamount of CO₂ introduced into the cathode inlet was roughly constant.The temperature profile for the fuel cell at steady state is shown inthe MCFC Temps section. The Fuel LHV values section describes the fuelinputs and outputs for the fuel cell, including the Reformable FuelSurplus Ratio, which is a ratio of reformable fuel delivered to theanode inlet relative to the amount of hydrogen consumed in the anode forgenerating electricity. The Energy Balance section shows the amount offuel introduced into either the fuel cell anode (for reforming and/orelectric power generation in the anode) and/or into the burner (foradding heat to the fuel cell). Additionally, the net energy output ofthe fuel cell is shown. It is noted that, because the model did notexplicitly model heat loss, the heat shown in the Energy Balance sectionrepresents a heat based on the difference between the generated voltageand the ideal voltage for the fuel cell, or waste heat. The values inthe Energy Balance section are shown in MegaWatt-Hours, for ease ofcomparison of energy values in the simulation.

In FIG. 7, the first column of numbers shows the values for baseline or“balanced” operation of a fuel cell using a configuration similar toFIG. 6. In the baseline configuration, the CO₂ utilization was about 73%while the fuel utilization in the anode was about 70%. The remainingcolumns show the impact of reducing the fuel utilization for a cathodeinlet stream having a roughly constant amount of CO₂. Although the fuelutilization appeared to have some impact on the cathode utilization,even at fuel utilization values of about 50% or less, a CO₂ utilizationof greater than about 60% was achieved. This demonstrated that theconfiguration similar to the configuration shown in FIG. 1 allowed forreduction in the fuel utilization to enhance power generation whilestill maintaining a high percentage of CO₂ utilization. Columns 4-6 alsoshow that a reformable surplus ratio of at least about 2.0 was achievedwhile maintaining a CO₂ utilization of at least about 60% and a fuelutilization of about 50% or less.

FIG. 8 shows results from additional simulations. The results shown inFIG. 8 appeared similar to those shown in FIG. 7, though an extra column(i.e., column 820) was included to show the changes in the simulationresults when parameters were adjusted to produce an output voltage of˜0.65V, rather than ˜0.72V. Column 802 lists row numbers for eachparameter. Column 804 identifies a parameter associated with a row.Column 806 identifies units in which the parameter is expressed, whenapplicable. Columns 808, 810, 812, 814, 816, 818, and 820 each show theresults of a different simulation based on different fuel cellconditions.

The fuel cell parameters in FIG. 8 were grouped into several sections.The Fuel Cell Parameters section 832 shows various initial parametersfor the simulation. The CO₂ Flow section 830 shows the values used todetermine the CO₂ utilization in each simulation. For the simulationshown in FIG. 8, the amount of CO₂ introduced into the cathode inlet wasroughly constant.

For the simulation results shown in columns 808, 810, 812, 814, 816, and818 operational parameters were selected to maintain an output voltageof ˜0.72V. In the simulation shown in column 820, operational parameterswere selected to maintain an output voltage of ˜0.65V.

The MCFC Temperature section 834 shows temperature parameters for thefuel cell at steady state. For these simulations, the anode inlettemperature (row 18) was held approximately constant at ˜536° C. TheReformable Fuel section 836 describes the fuel inputs and outputs forthe fuel cell, including the Reformable Fuel Surplus Ratio (row 17),which is a ratio of reformable fuel delivered to the anode inletrelative to the amount of hydrogen consumed in the anode for generatingelectricity.

The Energy Balance section 838 shows the amount of fuel introduced intothe fuel cell anode (for reforming and/or electric power generation inthe anode) and/or into the burner (for adding heat to the fuel cell).Additionally, the net energy output (row 28) of the fuel cell is shown.It is noted that, because the model did not explicitly model heat loss,the heat shown in the Energy Balance section 838 represents a heat basedon the difference between the generated voltage and the ideal voltagefor the fuel cell, or waste heat. The values in the Energy Balancesection are shown in MegaWatt-Hours, for ease of comparison of energyvalues in the simulation.

In FIG. 8, column 808 shows the values for baseline or “balanced”operation of a fuel cell using a configuration similar to FIG. 6. In thebaseline configuration, the CO₂ utilization was about 73% while the fuelutilization in the anode was about 70%. The remaining columns show theimpact of reducing the fuel utilization for a cathode inlet streamhaving a roughly constant amount of CO₂. Although the fuel utilizationcan have some impact on the cathode utilization, even at fuelutilization values of about 50% or less, a CO₂ utilization of greaterthan about 60% was achieved. This demonstrated that the configurationsimilar to the configuration shown in FIG. 1 allowed for reduction inthe fuel utilization to enhance power generation while still maintaininga high percentage of CO₂ utilization. Columns 814, 816, 818, and 820each show that a reformable surplus ratio of at least about 2.0 wasachieved while maintaining a CO₂ utilization of at least about 60% and afuel utilization of about 50% or less.

The CO₂ Sequestration section 840 shows the amount and % of CO₂ capturedin the different simulations. The Cathode Inlet section 842 showsparameters of the cathode inlet flow. The Cathode Outlet section 844shows parameters of the cathode outlet flow.

FIG. 9 shows results from additional simulations. The simulations shownin FIG. 9 used the exhaust from a gas turbine as the cathode inlet, atleast in part. The gas turbine had a CO₂ concentration of about 4.5%, incontrast to the CO₂ concentration of about 14.9% in the simulations ofFIG. 8. The overall system may be similar to the configuration shown inFIG. 1 with the burner replaced by a gas turbine and/or provided inaddition to the burner. Notice that no fuel was fed to the burner inthese simulations. Other arrangements where at least part of the gasturbine exhaust was supplied to the fuel cell cathode inlet arepossible. Column 920 was included to show the changes in the simulationresults when parameters were adjusted to produce an output voltage of˜0.65V, rather than ˜0.72V.

Column 902 lists row numbers for each parameter. Column 904 identifies aparameter associated with a row. Column 906 identifies units in whichthe parameter is expressed, when applicable. Columns 908, 910, 912, 914,916, 918, and 920 each show the results of a different simulation basedon different fuel cell conditions.

The fuel cell parameters in FIG. 9 are grouped into several sections.The CO₂ Utilization section 330 show the values used to determine theCO₂ utilization in each simulation. For the simulations in FIG. 9, theamount of CO₂ introduced into the cathode inlet (row1) was held roughlyconstant.

The Fuel Cell Parameters section 932 shows various initial parametersfor the simulation. For the simulations shown in columns 908, 910, 912,914, 916, and 918, operational parameters were selected to maintain anoutput voltage of ˜0.72V. In the simulation shown in column 920,operational parameters were selected to maintain an output voltage of˜0.65V.

The MCFC Temperature section 936 shows temperature parameters for thefuel cell at steady state. For these simulations, the anode inlettemperature (row 18) was held roughly constant at ˜536° C.

The Reformable Fuel section 934 describes the fuel inputs and outputsfor the fuel cell, including the Reformable Fuel Surplus Ratio (row 17),which is a ratio of reformable fuel delivered to the anode inletrelative to the amount of hydrogen consumed in the anode for generatingelectricity.

The Energy Balance section 938 shows the amount of fuel introduced intothe fuel cell anode (for reforming and/or electric power generation inthe anode) and/or into the burner (for adding heat to the fuel cell).Additionally, the net energy output of the fuel cell is shown. It isnoted that, because the model did not explicitly model heat loss, theheat shown in the Energy Balance section 938 represents a heat based onthe difference between the generated voltage and the ideal voltage forthe fuel cell, or waste heat. The values in the Energy Balance sectionare shown in MegaWatt-Hours, for ease of comparison of energy values inthe simulation.

In FIG. 9, column 908 shows the values for baseline or “balanced”operation of a fuel cell using a configuration similar to FIG. 6. In thebaseline configuration, the CO₂ utilization was about 73% while the fuelutilization in the anode was about 70%. The remaining columns show theimpact of reducing the fuel utilization for a cathode inlet stream thathad a roughly constant amount of CO₂. Although the fuel utilization hadsome impact on the cathode utilization, even at fuel utilization valuesof about 50% or less, a CO₂ utilization of greater than about 60% wasachieved. This demonstrated that the configuration similar to theconfiguration shown in FIG. 1 can allow for reduction in the fuelutilization to enhance power generation while still maintaining a highpercentage of CO₂ utilization. Columns 914, 916, 918, and 920 each showthat a reformable surplus ratio of at least about 2.0 was achieved whilemaintaining a CO₂ utilization of at least about 60% and a fuelutilization of about 50% or less.

The CO₂ Sequestration section 840 shows the amount and % of CO₂ capturedin the different simulations. The Cathode Inlet section 842 showsparameters of the cathode inlet flow. The Cathode Outlet section 844shows parameters of the cathode outlet flow.

FIG. 10 shows results from additional simulations. The simulations shownin FIG. 10 were run at a higher average anode temperature than thesimulations shown in FIGS. 7, 8, and 9. The simulations of FIG. 10 alsomaintained a substantially constant average temperature in the anodewhile varying the anode input temperature. In contrast, the simulationsof FIGS. 7, 8, and 9 held the anode input temperature constant, whilethe anode average temperature varied. Column 1002 lists row numbers foreach parameter. Column 1004 identifies a parameter associated with arow. Column 1006 identifies units in which the parameter is expressed,when applicable. Columns 1008, 1010, 1012, 1014, 1016, 1018, 1020, 1022,and 1024 each show the results of a different simulation based ondifferent fuel cell conditions.

The fuel cell parameters in FIG. 10 are grouped into several sections.The CO₂ utilization section 1030 show the values used to determine theCO₂ utilization in each simulation. For the simulations in FIG. 10, theamount of CO₂ introduced into the cathode inlet was held roughlyconstant.

The Fuel Cell Parameters section 1032 shows various initial parametersfor the simulation. For the simulations shown in columns 1008, 1010,1012, 1014, 1016, 1018, and 1024, operational parameters were selectedto maintain an output voltage of ˜0.72V. In the simulation shown incolumn 1020, operational parameters were selected to maintain an outputvoltage of ˜0.70V. In the simulation shown in column 1022, operationalparameters were selected to maintain an output voltage of ˜0.65V.

The Reformable Fuel section 1034 describes the fuel inputs and outputsfor the fuel cell, including the Reformable Fuel Surplus Ratio (row17),which is a ratio of reformable fuel delivered to the anode inletrelative to the amount of hydrogen consumed in the anode for generatingelectricity.

The MCFC Temperature section 1036 shows temperature parameters for thefuel cell at steady state. For these simulations, the anode averagetemperature (row 20) was held roughly constant at ˜650° C.

The Energy Balance section 1038 shows the amount of fuel introduced intothe fuel cell anode (for reforming and/or electric power generation inthe anode) and/or into the burner (for adding heat to the fuel cell).Additionally, the net energy output of the fuel cell is shown. It isnoted that, because the model did not explicitly model heat loss, theheat shown in the Energy Balance section represents a heat based on thedifference between the generated voltage and the ideal voltage for thefuel cell, or waste heat. The values in the Energy Balance section areshown in MegaWatt-Hours, for ease of comparison of energy values in thesimulation.

In FIG. 10, column 1008 shows the values for baseline or “balanced”operation of a fuel cell using a configuration similar to FIG. 6. In thebaseline configuration, the CO₂ utilization was about 73% while the fuelutilization in the anode was about 70%. The remaining columns show theimpact of reducing the fuel utilization for a cathode inlet streamhaving a roughly constant amount of CO₂. Although the fuel utilizationhad some impact on the cathode utilization, even at fuel utilizationvalues of about 50% or less, a CO₂ utilization of greater than about 60%was achieved. This demonstrated that the configuration similar to theconfiguration shown in FIG. 1 can allow for reduction in the fuelutilization to enhance power generation while still maintaining a highpercentage of CO₂ utilization. Columns 1014, 1016, 1018, and 1020 eachshow that a reformable surplus ratio of at least about 2.0 can beachieved while maintaining a CO₂ utilization of at least about 60% and afuel utilization of about 50% or less.

Benzene vs. Methane Example

Many of the above examples and discussion use CH₄ as the fuel for theanode. Aspects of the invention are not limited to use with CH₄. Forexample, in one aspect benzene could be provided as a reformable fuel. Aseries of simulations were performed to illustrate the performance of amolten carbonate fuel cell according to the invention using benzene. Thesimulations were based on the arrangement described above with referenceto FIG. 1. FIG. 11 depicts the MCFC's simulated CO₂ utilization in thecathode at different fuel utilizations for benzene and CH₄. The x-axis1110 shows fuel utilization and the y-axis 1120 shows the CO₂utilization in the cathode.

Plot 1130 shows CO₂ utilization in the cathode at different fuelutilization using benzene as a fuel. Plot 1132 shows the CO₂ utilizationin the cathode at different fuel utilization using CH₄ as a fuel. As canbe seen, the CO₂ utilization in the cathode at different fuelutilizations appeared to differ significantly for benzene and CH₄.

Although the present invention has been described in terms of specificembodiments, it is not so limited. Suitable alterations/modificationsfor operation under specific conditions should be apparent to thoseskilled in the art. It is therefore intended that the following claimsbe interpreted as covering all such alterations/modifications that fallwithin the true spirit/scope of the invention.

What is claimed is:
 1. A method for producing electricity, and hydrogenor syngas, using a molten carbonate fuel cell comprising an anode and acathode, the method comprising: introducing an anode fuel streamcomprising a reformable fuel into the anode of the molten carbonate fuelcell, an internal reforming element associated with the anode of themolten carbonate fuel cell, or a combination thereof; introducing acathode inlet stream comprising CO₂ and O₂ into the cathode of themolten carbonate fuel cell; generating electricity within the moltencarbonate fuel cell; generating an anode exhaust from an anode outlet ofthe molten carbonate fuel cell; separating from the anode exhaust aH₂-containing stream, a syngas-containing stream, or a combinationthereof, wherein a fuel utilization of the anode of the molten carbonatefuel cell is about 50% or less and a CO₂ utilization of the cathode ofthe molten carbonate fuel cell is at least about 60%.
 2. The method ofclaim 1, wherein a reformable hydrogen content of the reformable fuelintroduced into the anode of the molten carbonate fuel cell, theinternal reforming element associated with the anode of the moltencarbonate fuel cell, or the combination thereof, is at least about 75%greater than an amount of H₂ oxidized in the anode of the moltencarbonate fuel cell to generate electricity.
 3. The method of claim 1,wherein the cathode inlet stream comprises about 20 vol % CO₂ or less.4. The method of claim 1, wherein the fuel utilization of the anode ofthe molten carbonate fuel cell is about 40% or less.
 5. The method ofclaim 1, wherein the CO₂ utilization of the cathode of the moltencarbonate fuel cell is at least about 65%.
 6. The method of claim 1,wherein the anode fuel stream comprises at least about 10 vol % inertcompounds, at least about 10 vol % CO₂, or a combination thereof.
 7. Themethod of claim 1, wherein the syngas-containing stream has a molarratio of H₂ to CO from about 3.0:1 to about 1.0:1.
 8. The method ofclaim 7, wherein the syngas-containing stream has a molar ratio of H₂ toCO from about 2.5:1 to about 1.5:1, or a combination thereof.
 9. Themethod of claim 1, wherein the anode exhaust has a molar ratio of H₂ toCO of about 1.5:1 to about 10:1.
 10. The method of claim 9, wherein theanode exhaust has a molar ratio of H₂ to CO of at least about 3.0:1. 11.The method of claim 1, wherein less than 10 vol % of H₂ produced in theanode of the molten carbonate fuel cell in a single pass is directly orindirectly recycled to the anode of the molten carbonate fuel cell orthe cathode of the molten carbonate fuel cell.
 12. The method of claim1, wherein less than 10 vol % of the syngas-containing stream isdirectly or indirectly recycled to the anode of the molten carbonatefuel cell or the cathode of the molten carbonate fuel cell.
 13. Themethod of claim 1, wherein less than 10 vol % of the anode exhaust isdirectly or indirectly recycled to the anode of the molten carbonatefuel cell or the cathode of the molten carbonate fuel cell.
 14. Themethod of claim 1, wherein no portion of the anode exhaust is directlyor indirectly recycled to the anode of the molten carbonate fuel cell,directly or indirectly recycled to the cathode of the molten carbonatefuel cell, or a combination thereof.
 15. The method of claim 1, furthercomprising separating at least one of CO₂ and H₂O from one or acombination of i) the anode exhaust, ii) the H₂-containing stream, andiii) the syngas-containing stream.
 16. The method of claim 1, whereinthe H₂-containing stream contains at least about 90 vol % H₂.
 17. Themethod of claim 1, wherein the cathode inlet stream comprises acombustion exhaust stream from a combustion-powered generator.
 18. Themethod of claim 1, wherein the molten carbonate fuel cell is operated ata voltage V_(A) of about 0.67 Volts or less.